tstr, conf: Ensure error strings are nul-terminated

This issue was found with oss-fuzz.

  ==18420==WARNING: MemorySanitizer: use-of-uninitialized-value
	      #0 0x55dd798a5797 in nxt_vsprintf unit/src/nxt_sprintf.c:163:31
	      #1 0x55dd798d5bdb in nxt_conf_vldt_error unit/src/nxt_conf_validation.c:1525:11
	      #2 0x55dd798dd4cd in nxt_conf_vldt_var unit/src/nxt_conf_validation.c:1560:16
	      #3 0x55dd798dd4cd in nxt_conf_vldt_if unit/src/nxt_conf_validation.c:1592:16
	      nginx#4 0x55dd798d55f4 in nxt_conf_vldt_object unit/src/nxt_conf_validation.c:2815:23
	      nginx#5 0x55dd798d6f84 in nxt_conf_vldt_access_log unit/src/nxt_conf_validation.c:3426:11
	      nginx#6 0x55dd798d55f4 in nxt_conf_vldt_object unit/src/nxt_conf_validation.c:2815:23
	      nginx#7 0x55dd798d47bd in nxt_conf_validate unit/src/nxt_conf_validation.c:1421:11
	      nginx#8 0x55dd79871c82 in LLVMFuzzerTestOneInput unit/fuzzing/nxt_json_fuzz.c:67:5
	      nginx#9 0x55dd79770620 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:614:13
	      nginx#10 0x55dd7975adb4 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:327:6
	      nginx#11 0x55dd7976084a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:862:9
	      nginx#12 0x55dd7978cc42 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
	      nginx#13 0x7e8192213082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/libc-start.c:308:16
	      nginx#14 0x55dd7975188d in _start

	    Uninitialized value was created by an allocation of 'error.i' in the stack frame
	      #0 0x55dd798dd42b in nxt_conf_vldt_var unit/src/nxt_conf_validation.c:1557:5
	      #1 0x55dd798dd42b in nxt_conf_vldt_if unit/src/nxt_conf_validation.c:1592:16

The issue was in nxt_tstr_test() where we create an error message with
nxt_sprintf(), where this error message is then later used with the
'%s' format specifier which expects a nul-terminated string, but by
default nxt_sprintf() doesn't nul-terminate, you must use the '%Z'
specifier to signify a '\0' at the end of the string.

Link: <https://github.com/google/oss-fuzz>
Link: <https://oss-fuzz.com/testcase-detail/5545917827055616>
Co-developed-by: Zhidao HONG <[email protected]>
Signed-off-by: Zhidao HONG <[email protected]>
Reviewed-by: Andrew Clayton <[email protected]>
Signed-off-by: Arjun <[email protected]>
[ Commit message/subject - Andrew ]
Signed-off-by: Andrew Clayton <[email protected]>

src/nxt_tstr.c

@@ -159,7 +159,7 @@ nxt_tstr_test(nxt_tstr_state_t *state, nxt_str_t *str, u_char *error)
 #else
         nxt_sprintf(error, error + NXT_MAX_ERROR_STR,
                     "Unit is built without support of njs: "
-                    "\"--njs\" ./configure option is missing.");
+                    "\"--njs\" ./configure option is missing.%Z");
         return NXT_ERROR;
 #endif