pop3_doing: don't call pop3_dophase_done() if already failed

... it also clobbered the 'result' return value so that it wouldn't
return the error back to the parent function properly, which broke test
809 when run with 'multi-always'.

lib/pop3.c

@@ -1554,7 +1554,7 @@ static CURLcode pop3_doing(struct connectdata *conn, bool *dophase_done)
   CURLcode result;
   result = pop3_multi_statemach(conn, dophase_done);
 
-  if(*dophase_done) {
+  if(!result && *dophase_done) {
     result = pop3_dophase_done(conn, FALSE /* not connected */);
 
     DEBUGF(infof(conn->data, "DO phase is complete\n"));