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/osdep/unix/config.c

@@ -89,7 +89,7 @@ grub_util_load_config (struct grub_util_config *cfg)
   argv[0] = "sh";
   argv[1] = "-c";
 
-  script = xmalloc (4 * strlen (cfgfile) + 300);
+  script = xcalloc (4, strlen (cfgfile) + 300);
 
   ptr = script;
   memcpy (ptr, ". '", 3);