Fix UBSAN reported by Prof. Ripley

[bobjansen]

UBSAN has recently been found, as described here and quoted below for reference:

R Under development (unstable) (2022-04-07 r82109) -- "Unsuffered Consequences"
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> pkgname <- "scrypt"
> source(file.path(R.home("share"), "R", "examples-header.R"))
> options(warn = 1)
> library('scrypt')
> 
> base::assign(".oldSearch", base::search(), pos = 'CheckExEnv')
> base::assign(".old_wd", base::getwd(), pos = 'CheckExEnv')
> cleanEx()
> nameEx("hashPassword")
> ### * hashPassword
> 
> flush(stderr()); flush(stdout())
> 
> ### Name: hashPassword
> ### Title: Hash a password
> ### Aliases: hashPassword
> 
> ### ** Examples
> 
> # Hash password using default parameters
> hashPassword('passw0rd')
scrypt-1.1.6/lib/crypto/sha256.c:254:24: runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:44:28: note: nonnull attribute specified here
    #0 0x7f40b5fa730f in scrypt_SHA256_Update /data/gannet/ripley/R/packages/tests-clang-SAN/scrypt/src/scrypt-1.1.6/lib/crypto/sha256.c:254:3
    #1 0x7f40b5fc52ab in scrypt_HMAC_SHA256_Update /data/gannet/ripley/R/packages/tests-clang-SAN/scrypt/src/scrypt-1.1.6/lib/crypto/sha256.c:335:2
    #2 0x7f40b5fc52ab in PBKDF2_SHA256 /data/gannet/ripley/R/packages/tests-clang-SAN/scrypt/src/scrypt-1.1.6/lib/crypto/sha256.c:377:2
    #3 0x7f40b5fc5b4b in crypto_scrypt /data/gannet/ripley/R/packages/tests-clang-SAN/scrypt/src/scrypt-1.1.6/lib/crypto/crypto_scrypt-ref.c:258:2
    #4 0x7f40b5ff3bfd in getcpuperf(double*) /data/gannet/ripley/R/packages/tests-clang-SAN/scrypt/src/util.cpp:145:13
    #5 0x7f40b5fede5d in (anonymous namespace)::getparams(double, double, int*, unsigned int*, unsigned int*) /data/gannet/ripley/R/packages/tests-clang-SAN/scrypt/src/scrypt.cpp:49:15
    #6 0x7f40b5fed46f in hashPassword(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, double, double) /data/gannet/ripley/R/packages/tests-clang-SAN/scrypt/src/scrypt.cpp:129:15
    #7 0x7f40b5fcb821 in _scrypt_hashPassword /data/gannet/ripley/R/packages/tests-clang-SAN/scrypt/src/RcppExports.cpp:17:34
    #8 0x6e6372 in R_doDotCall /data/gannet/ripley/R/svn/R-devel/src/main/dotcode.c:607:17
    #9 0x84bf7e in bcEval /data/gannet/ripley/R/svn/R-devel/src/main/eval.c:7692:21
    #10 0x82d14e in Rf_eval /data/gannet/ripley/R/svn/R-devel/src/main/eval.c:748:8
    #11 0x8959c3 in R_execClosure /data/gannet/ripley/R/svn/R-devel/src/main/eval.c
    #12 0x89166f in Rf_applyClosure /data/gannet/ripley/R/svn/R-devel/src/main/eval.c:1844:16
    #13 0x82db88 in Rf_eval /data/gannet/ripley/R/svn/R-devel/src/main/eval.c:871:12
    #14 0x95c7d6 in Rf_ReplIteration /data/gannet/ripley/R/svn/R-devel/src/main/main.c:264:2
    #15 0x95fd30 in R_ReplConsole /data/gannet/ripley/R/svn/R-devel/src/main/main.c:316:11
    #16 0x95fb39 in run_Rmainloop /data/gannet/ripley/R/svn/R-devel/src/main/main.c:1137:5
    #17 0x95fe72 in Rf_mainloop /data/gannet/ripley/R/svn/R-devel/src/main/main.c:1144:5
    #18 0x4f30ba in main /data/gannet/ripley/R/svn/R-devel/src/main/Rmain.c:29:5
    #19 0x7f40c5cc2b74 in __libc_start_main (/lib64/libc.so.6+0x27b74) (BuildId: 08df60634339b221bb854d4e10b7278cafde70c4)
    #20 0x43231d in _start (/data/gannet/ripley/R/R-clang-SAN/bin/exec/R+0x43231d)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior scrypt-1.1.6/lib/crypto/sha256.c:254:24 in 
[1] "c2NyeXB0AA0AAAAIAAAAAVYLCibsie4sa+qsjl+/M91oAQHaF/cbdCXgkKVHbqjy3Oa6wR0drHouY8rxnjYgDolqGcDhAEE4Wrg7G0geuC4M/xoaVRHhX+fWnzIUTpGj"
> 
> # Hash password with custom parameters
> hashPassword('passw0rd', maxmem=0.25, maxtime=1.0)
[1] "c2NyeXB0AA0AAAAIAAAAAWpuqY05OCSG48XLNnakPpaBjkLwviKQ/JAI85CmY5hgtUR4FBVx9ac1bf/cvgA8tZWL7cpo40MRpMpWodBaEnTK1VqLxdi9v0VUGW4EW6At"
> 
> 
> 
> cleanEx()
> nameEx("verifyPassword")
> ### * verifyPassword
> 
> flush(stderr()); flush(stdout())
> 
> ### Name: verifyPassword
> ### Title: Verify a hashed password
> ### Aliases: verifyPassword
> 
> ### ** Examples
> 
> # Hash password using default parameters
> hashed <- hashPassword("password")
> 
> # verify invalid password
> verifyPassword(hashed, "bad password");
[1] FALSE
> 
> # verify correct password
> verifyPassword(hashed, "password")
[1] TRUE
> 
> 
> 
> ### * <FOOTER>
> ###
> cleanEx()
> options(digits = 7L)
> base::cat("Time elapsed: ", proc.time() - base::get("ptime", pos = 'CheckExEnv'),"\n")
Time elapsed:  4.176 0.134 12.129 0 0 
> grDevices::dev.off()
null device 
          1 
> ###
> ### Local variables: ***
> ### mode: outline-minor ***
> ### outline-regexp: "\\(> \\)?### [*]+" ***
> ### End: ***
> quit('no')