builtin-mv: fix use of uninitialized memory.

Juergen Ruehle noticed that add_slash() tries to strcat()
into uninitialized memory and fails.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-mv.c

@@ -48,7 +48,8 @@ static const char *add_slash(const char *path)
 	if (path[len - 1] != '/') {
 		char *with_slash = xmalloc(len + 2);
 		memcpy(with_slash, path, len);
-		strcat(with_slash + len, "/");
+		with_slash[len++] = '/';
+		with_slash[len] = 0;
 		return with_slash;
 	}
 	return path;