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/i386/pc/vga.c

@@ -127,7 +127,7 @@ grub_video_vga_setup (unsigned int width, unsigned int height,
 
   vga_height = height ? : 480;
 
-  framebuffer.temporary_buffer = grub_malloc (vga_height * VGA_WIDTH);
+  framebuffer.temporary_buffer = grub_calloc (vga_height, VGA_WIDTH);
   framebuffer.front_page = 0;
   framebuffer.back_page = 0;
   if (!framebuffer.temporary_buffer)