Fix #7: inconsistent return value in mkpath()

When the required `dir` argument is missing `mkpath()` inconsistently
returns 1 instead of -1.  Found by @jesponde

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

makepath.c

@@ -35,7 +35,7 @@ int mkpath(char *dir, mode_t mode)
 
 	if (!dir) {
 		errno = EINVAL;
-		return 1;
+		return -1;
 	}
 
 	if (!stat(dir, &sb))