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/partmap/gpt.c

@@ -199,7 +199,7 @@ gpt_partition_map_embed (struct grub_disk *disk, unsigned int *nsectors,
   *nsectors = ctx.len;
   if (*nsectors > max_nsectors)
     *nsectors = max_nsectors;
-  *sectors = grub_malloc (*nsectors * sizeof (**sectors));
+  *sectors = grub_calloc (*nsectors, sizeof (**sectors));
   if (!*sectors)
     return grub_errno;
   for (i = 0; i < *nsectors; i++)