calloc: Use calloc() at most places

This modifies most of the places we do some form of:

  X = malloc(Y * Z);

to use calloc(Y, Z) instead.

Among other issues, this fixes:
  - allocation of integer overflow in grub_png_decode_image_header()
    reported by Chris Coulson,
  - allocation of integer overflow in luks_recover_key()
    reported by Chris Coulson,
  - allocation of integer overflow in grub_lvm_detect()
    reported by Chris Coulson.

Fixes: CVE-2020-14308

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

grub-core/commands/search_wrap.c

@@ -122,7 +122,7 @@ grub_cmd_search (grub_extcmd_context_t ctxt, int argc, char **args)
     for (i = 0; state[SEARCH_HINT_BAREMETAL].args[i]; i++)
       nhints++;
 
-  hints = grub_malloc (sizeof (hints[0]) * nhints);
+  hints = grub_calloc (nhints, sizeof (hints[0]));
   if (!hints)
     return grub_errno;
   j = 0;