libobs: Handle empty path in os_get_path_extension

os_readdir might return an empty path if the OS filename doesn't fit
into the buffer after os_wcs_to_utf8. If this empty path is then passed
to os_get_path_extension it causes a crash.

libobs/util/platform.c

@@ -653,6 +653,9 @@ const char *os_get_path_extension(const char *path)
 	char *period;
 	char *slash;
 
+	if (!path[0])
+		return NULL;
+
 	dstr_init_copy(&temp, path);
 	dstr_replace(&temp, "\\", "/");