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/lib/libgcrypt/cipher/primegen.c

@@ -383,7 +383,7 @@ prime_generate_internal (int need_q_factor,
     }
 
   /* Allocate an array to track pool usage. */
-  pool_in_use = gcry_malloc (n * sizeof *pool_in_use);
+  pool_in_use = gcry_calloc (n, sizeof *pool_in_use);
   if (!pool_in_use)
     {
       err = gpg_err_code_from_errno (errno);
@@ -765,7 +765,7 @@ gen_prime (unsigned int nbits, int secret, int randomlevel,
   if (nbits < 16)
     log_fatal ("can't generate a prime with less than %d bits\n", 16);
 
-  mods = gcry_xmalloc( no_of_small_prime_numbers * sizeof *mods );
+  mods = gcry_xcalloc( no_of_small_prime_numbers, sizeof *mods);
   /* Make nbits fit into gcry_mpi_t implementation. */
   val_2  = mpi_alloc_set_ui( 2 );
   val_3 = mpi_alloc_set_ui( 3);