nios2: fix bootm error on fdt args

We should check argv[3] only if there are enough args. Otherwise,
it might cause invalid memory access fault.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>

arch/nios2/lib/bootm.c

@@ -42,7 +42,7 @@ int do_bootm_linux(int flag, int argc, char * const argv[], bootm_headers_t *ima
 	if (boot_get_fdt(flag, argc, argv, images, &of_flat_tree, &of_size))
 		return 1;
 #endif
-	if (!of_flat_tree)
+	if (!of_flat_tree && argc > 3)
 		of_flat_tree = (char *)simple_strtoul(argv[3], NULL, 16);
 	if (of_flat_tree)
 		initrd_end = (ulong)of_flat_tree;