multi: set CURLM_CALL_MULTI_PERFORM after switch to DOING_MORE

Since there is nothing to wait for there. Avoids the test 1233 hang
reported in #12033.

Reported-by: Dan Fandrich
Closes #12042

lib/multi.c

@@ -2221,6 +2221,7 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
             /* we're supposed to do more, but we need to sit down, relax
                and wait a little while first */
             multistate(data, MSTATE_DOING_MORE);
+            rc = CURLM_CALL_MULTI_PERFORM;
           }
           else {
             /* we're done with the DO, now DID */