In PQsendQueryStart(), avoid leaking any left-over async result.

Ordinarily there would not be an async result sitting around at this
point, but it appears that in corner cases there can be.  Considering
all the work we're about to launch, it's hardly going to cost anything
noticeable to check.

It's been like this forever, so back-patch to all supported branches.

Report: <CAD-Qf1eLUtBOTPXyFQGW-4eEsop31tVVdZPu4kL9pbQ6tJPO8g@mail.gmail.com>

src/interfaces/libpq/fe-exec.c

@@ -1386,8 +1386,7 @@ PQsendQueryStart(PGconn *conn)
 	}
 
 	/* initialize async result-accumulation state */
-	conn->result = NULL;
-	conn->next_result = NULL;
+	pqClearAsyncResult(conn);
 
 	/* reset single-row processing mode */
 	conn->singleRowMode = false;