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/video/emu/sdl.c

@@ -172,7 +172,7 @@ grub_video_sdl_set_palette (unsigned int start, unsigned int count,
       if (start + count > mode_info.number_of_colors)
 	count = mode_info.number_of_colors - start;
 
-      tmp = grub_malloc (count * sizeof (tmp[0]));
+      tmp = grub_calloc (count, sizeof (tmp[0]));
       for (i = 0; i < count; i++)
 	{
 	  tmp[i].r = palette_data[i].r;