part_efi: In is_gpt_valid() check argument validity before allocation

While this goes somewhat against normal coding style we should ensure
that dev_desc is not NULL before we dereference it in allocation of
legacy_mbr.

Reported-by: Coverity (CID: 167292)
Signed-off-by: Tom Rini <trini@konsulko.com>

disk/part_efi.c

@@ -923,13 +923,14 @@ static int is_pmbr_valid(legacy_mbr * mbr)
 static int is_gpt_valid(struct blk_desc *dev_desc, u64 lba,
 			gpt_header *pgpt_head, gpt_entry **pgpt_pte)
 {
-	ALLOC_CACHE_ALIGN_BUFFER(legacy_mbr, mbr, dev_desc->blksz);
-
+	/* Confirm valid arguments prior to allocation. */
 	if (!dev_desc || !pgpt_head) {
 		printf("%s: Invalid Argument(s)\n", __func__);
 		return 0;
 	}
 
+	ALLOC_CACHE_ALIGN_BUFFER(legacy_mbr, mbr, dev_desc->blksz);
+
 	/* Read MBR Header from device */
 	if (blk_dread(dev_desc, 0, 1, (ulong *)mbr) != 1) {
 		printf("*** ERROR: Can't read MBR header ***\n");