lib-index: zero mmap_length after logging error

Logging that mmap(size=0) failed looks suspicious.

src/lib-index/mail-cache.c

@@ -480,12 +480,12 @@ int mail_cache_map(struct mail_cache *cache, size_t offset, size_t size,
 	cache->mmap_base = mmap_ro_file(cache->fd, &cache->mmap_length);
 	if (cache->mmap_base == MAP_FAILED) {
 		cache->mmap_base = NULL;
-		cache->mmap_length = 0;
 		if (ioloop_time != cache->last_mmap_error_time) {
 			cache->last_mmap_error_time = ioloop_time;
 			mail_cache_set_syscall_error(cache, t_strdup_printf(
 				"mmap(size=%"PRIuSIZE_T")", cache->mmap_length));
 		}
+		cache->mmap_length = 0;
 		return -1;
 	}
 	*data_r = offset > cache->mmap_length ? NULL :