coreutils snapshot vs gcc7-to-be: new test-calloc-gnu.c and test-canonicalize.c warnings

[Jim Meyering]

Building the latest coreutils snapshot with gcc just built from its
own latest git, I hit this:

  test-calloc-gnu.c: In function 'main':
  test-calloc-gnu.c:32:5: warning: product '2305843009213693952 * 8'
of arguments 1 and 2 exceeds 'SIZE_MAX' [-Walloc-size-larger-than=]
     p = calloc ((size_t) -1 / 8 + 1, 8);
     ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  In file included from ../lib/stdlib.h:36:0,
                   from test-calloc-gnu.c:19:
  /usr/include/stdlib.h:467:14: note: in a call to allocation function
'calloc' declared here
   extern void *calloc (size_t __nmemb, size_t __size)
                ^~~~~~
and this:

test-canonicalize.c: In function 'main':
test-canonicalize.c:78:13: error: argument 1 null where non-null
expected [-Werror=nonnull]
     result1 = canonicalize_file_name (null_ptr ());
     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ../lib/stdlib.h:36:0,
                 from ../lib/canonicalize.h:20,
                 from test-canonicalize.c:21:
/usr/include/stdlib.h:723:14: note: in a call to function
'canonicalize_file_name' declared here
 extern char *canonicalize_file_name (const char *__name)
              ^~~~~~~~~~~~~~~~~~~~~~