Fix allocation check to test the right variable

The memory allocation for cancelConn->be_cancel_key was accidentally
checking the be_cancel_key member in the conn object instead of the
one in cancelConn.

Author: Ranier Vilela <ranier.vf@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAEudQAq4ySDR6dsg9xwurBXwud02hX7XCOZZAcZx-JMn6A06nA@mail.gmail.com

src/interfaces/libpq/fe-cancel.c

@@ -114,7 +114,7 @@ PQcancelCreate(PGconn *conn)
 	if (conn->be_cancel_key != NULL)
 	{
 		cancelConn->be_cancel_key = malloc(conn->be_cancel_key_len);
-		if (!conn->be_cancel_key)
+		if (cancelConn->be_cancel_key == NULL)
 			goto oom_error;
 		memcpy(cancelConn->be_cancel_key, conn->be_cancel_key, conn->be_cancel_key_len);
 	}