Check request size against PTRDIFF_MAX before calling malloc,
calloc, and realloc. Otherwise, gcc 9 and later complain with
thie warning message.

In file included from var_getput.c:17:
var_getput.c: In function 'ncmpi_mput_var':
../../../PnetCDF/src/drivers/include/common.h:83:26: warning:
argument 1 range [18446744065119617024, 18446744073709551612] exceeds
maximum object size 9223372036854775807 [-Walloc-size-larger-than=]
   83 | #define NCI_Malloc(a)    malloc(a)
      |                          ^~~~~~~~~
var_getput.c:20555:19: note: in expansion of macro 'NCI_Malloc'
20555 |     reqs = (int*) NCI_Malloc(sizeof(int) * nvars);
      |                   ^~~~~~~~~~
In file included from var_getput.c:12:
/usr/include/stdlib.h:539:14: note: in a call to allocation function
'malloc' declared here
  539 | extern void *malloc (size_t __size) __THROW __attribute_malloc__ __wur;
      |              ^~~~~~