Tidy up error checking and fix bug in hash command

There are two problems:

1. The argument count needs to be checked before argv is used
2. When verify is not enabled, we need to define a constant zero value

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_hash.c b/common/cmd_hash.c
index 689c608..eb6a338 100644
--- a/common/cmd_hash.c
+++ b/common/cmd_hash.c
@@ -32,11 +32,15 @@
 #ifdef CONFIG_HASH_VERIFY
 	int verify = 0;
 
+	if (argc < 4)
+		return CMD_RET_USAGE;
 	if (!strcmp(argv[1], "-v")) {
 		verify = 1;
 		argc--;
 		argv++;
 	}
+#else
+	const int verify = 0;
 #endif
 	/* Move forward to 'algorithm' parameter */
 	argc--;