lib-index: Don't break indexes on syscall failures during index refre…

…shing.

Especially mmap() failures due to out of memory could have triggered this.
We treated the open as successful, which meant that an empty index was
opened.

src/lib-index/mail-index-sync-update.c

@@ -955,6 +955,11 @@ int mail_index_sync_map(struct mail_index_map **_map,
 					    &reset, &reason);
 	if (ret <= 0) {
 		mail_index_view_close(&view);
+		if (force && ret < 0) {
+			/* if we failed because of a syscall error, make sure
+			   we return a failure. */
+			return -1;
+		}
 		if (force && ret == 0) {
 			/* the seq/offset is probably broken */
 			mail_index_set_error(index, "Index %s: Lost log for "