idevicebackup2: Fix fd leak in mb2_copy_file_by_path()

fd was leak in error path.
diff --git a/tools/idevicebackup2.c b/tools/idevicebackup2.c
index e6c1d3f..010e51d 100644
--- a/tools/idevicebackup2.c
+++ b/tools/idevicebackup2.c
@@ -1150,6 +1150,7 @@
 	/* open destination file */
 	if ((to = fopen(dst, "wb")) == NULL) {
 		printf("Cannot open destination file '%s'.\n", dst);
+		fclose(from);
 		return;
 	}