calloc: Make sure we always have an overflow-checking calloc() available

This tries to make sure that everywhere in this source tree, we always have
an appropriate version of calloc() (i.e. grub_calloc(), xcalloc(), etc.)
available, and that they all safely check for overflow and return NULL when
it would occur.

Signed-off-by: Peter Jones <pjones@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/lib/posix_wrap/stdlib.h

@@ -21,6 +21,7 @@
 
 #include <grub/mm.h>
 #include <grub/misc.h>
+#include <grub/safemath.h>
 
 static inline void 
 free (void *ptr)
@@ -37,7 +38,12 @@ malloc (grub_size_t size)
 static inline void *
 calloc (grub_size_t size, grub_size_t nelem)
 {
-  return grub_zalloc (size * nelem);
+  grub_size_t sz;
+
+  if (grub_mul (size, nelem, &sz))
+    return NULL;
+
+  return grub_zalloc (sz);
 }
 
 static inline void *