New function xpalloc in module xalloc

[Marc Nieper-Wißkirchen]

GCC prints the following warning when compiling the new code:

lib/xmalloc.c: In function 'xpalloc':
lib/xmalloc.c:132:64: warning: comparison of integer expressions of
different signedness: 'long unsigned int' and 'idx_t' {aka 'long int'}
[-Wsign-compare]
  132 |     = ((INT_MULTIPLY_WRAPV (n, item_size, &nbytes) || SIZE_MAX <
nbytes)

If I understand the error message correctly, it is because 'nbytes' is a
signed type while SIZE_MAX is unsigned.

Does the comparison make any sense, by the way?

Thanks for investigating,

Marc