git-blame: prevent argument parsing segfault

The 3rd branch in builtin-blame.c should also check for lacking
arguments.  Running that in top dir does not trigger the problem
because the 'prefix' is NULL.

Signed-off-by: Tommi Kyntola <tommi.kyntola@ray.fi>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-blame.c

@@ -2200,6 +2200,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 			if (!strcmp(argv[j], "--"))
 				seen_dashdash = j;
 		if (seen_dashdash) {
+			/* (2) */
 			if (seen_dashdash + 1 != argc - 1)
 				usage(blame_usage);
 			path = add_prefix(prefix, argv[seen_dashdash + 1]);
@@ -2208,6 +2209,8 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 		}
 		else {
 			/* (3) */
+			if (argc <= i)
+				usage(blame_usage);
 			path = add_prefix(prefix, argv[i]);
 			if (i + 1 == argc - 1) {
 				final_commit_name = argv[i + 1];