fix missing initialization in tus_file_add()

this could lead to later assertion failures when the tus directory
contained failes not fit for loading (e.g. wrong size/magic etc)
because valid files would be munmap()ed.

src/tus_file.c

@@ -557,8 +557,8 @@ tus_file_unref(struct tus_file_core *fcore)
 static void
 tus_file_add(struct VPFX(tus_server) *srv, int basefd, const char *filename)
 {
-	struct tus_file_core *fcore;
-	struct tus_file_disk *fdisk;
+	struct tus_file_core *fcore = NULL;
+	struct tus_file_disk *fdisk = NULL;
 	struct stat st;
 	int fd = -1;
 	const char *err;