out_s3: fix NULL dereference when upload_id is unset

When upload_id is not set in create_multipart_upload() (which can occur
in some failure modes), subsequent retries will segfault when checking
this ID in complete_multipart_upload().

This commit simply checks that the upload ID has been set and returns
the usual error code instead of crashing if not. It also logs an error
message for the user.

Discussed in fluent#3838.

Signed-off-by: Aaron Jacobs <aaron.jacobs@crescendotechnology.com>

plugins/out_s3/s3_multipart.c

@@ -431,6 +431,12 @@ int complete_multipart_upload(struct flb_s3 *ctx,
     struct flb_http_client *c = NULL;
     struct flb_aws_client *s3_client;
 
+    if (!m_upload->upload_id) {
+        flb_plg_error(ctx->ins, "Cannot complete multipart upload for key %s: "
+                      "upload ID is unset ", m_upload->s3_key);
+        return -1;
+    }
+
     uri = flb_sds_create_size(flb_sds_len(m_upload->s3_key) + 11 +
                               flb_sds_len(m_upload->upload_id));
     if (!uri) {