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/disk/xen/xendisk.c

@@ -426,7 +426,7 @@ grub_xendisk_init (void)
   if (!ctr)
     return;
 
-  virtdisks = grub_malloc (ctr * sizeof (virtdisks[0]));
+  virtdisks = grub_calloc (ctr, sizeof (virtdisks[0]));
   if (!virtdisks)
     return;
   if (grub_xenstore_dir ("device/vbd", fill, &ctr))