Silence most -Wzero-as-null-pointer-constant diagnostics

Replace 0 by NULL and {0} by {}.

Omit a few cases that aren't so trivial to fix.

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>
Link: <https://software.codidact.com/posts/292718/292759#answer-292759>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

malloc/reallocarray.c

@@ -27,7 +27,7 @@ __libc_reallocarray (void *optr, size_t nmemb, size_t elem_size)
   if (__builtin_mul_overflow (nmemb, elem_size, &bytes))
     {
       __set_errno (ENOMEM);
-      return 0;
+      return NULL;
     }
   return realloc (optr, bytes);
 }