Fix for issue #810

[mborland]

Addresses 0 as null pointer constant and sign compare warnings as well.

[jzmaddock]

The failure is unconnected but worrying (GCC-9 and gun++11 mode):

testing.capture-output ../../../bin.v2/libs/math/test/test_chatterjee_correlation.test/gcc-9/debug/link-static/threading-multi/visibility-hidden/test_chatterjee_correlation.run

====== BEGIN OUTPUT ======

Error at test_chatterjee_correlation.cpp:test_paper:130:

  Mollified relative error in float precision is 0.0561, which exceeds 0.012, error/tol  = 4.68.

  Expected: +0.828877568 = +0x1.a862a4p-1

  Computed: +0.884999990 = +0x1.c51eb8p-1

  ULP distance: +941578

malloc(): invalid next size (unsorted)

Aborted (core dumped)

[jzmaddock]

At https://drone.cpp.al/boostorg/math/768/197/2

[jzmaddock]

See also #812 which extends this PR with further tests.
(no reason not to go ahead and merge this one though).

[mborland]

The failure is unconnected but worrying (GCC-9 and gun++11 mode):

testing.capture-output ../../../bin.v2/libs/math/test/test_chatterjee_correlation.test/gcc-9/debug/link-static/threading-multi/visibility-hidden/test_chatterjee_correlation.run

====== BEGIN OUTPUT ======

Error at test_chatterjee_correlation.cpp:test_paper:130:

  Mollified relative error in float precision is 0.0561, which exceeds 0.012, error/tol  = 4.68.

  Expected: +0.828877568 = +0x1.a862a4p-1

  Computed: +0.884999990 = +0x1.c51eb8p-1

  ULP distance: +941578

malloc(): invalid next size (unsorted)

Aborted (core dumped)

I think that test can be safely removed from CI testing. I'll look at it this week and open a PR for it.