mmap/mmap: Fix integer overflow in binary search

The integer overflow triggered for simple masks in the "badram"
command, such as "badram 0x0000000012340000,0xfffffffffffffff8".
This resulted in an infinite loop, locking up the machine.

Signed-off-by: Wanda Phinode <wanda@phinode.net>
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Author: wanda-phi

grub-core/mmap/mmap.c

@@ -409,7 +409,7 @@ badram_iter (grub_uint64_t addr, grub_uint64_t size,
       */
       while (high - low > 1)
 	{
-	  cur = (low + high) / 2;
+	  cur = low + (high - low) / 2;
 	  if (fill_mask (entry, cur) >= addr)
 	    high = cur;
 	  else