MXS-3425: Fix handling of LOAD DATA LOCAL INFILE

The LOAD DATA LOCAL INFILE is handled in a way where it returns two
results that both are complete: the first one with the file being
requested and the second one with the final OK packet. Readwritesplit
called session_book_server_response for both statements which caused the
current query index to drop to -1 which in turn was unconditionally used
as the buffer offset.

The new check for the invalid index value will help prevent crashes in
production while still allowing it to be detected while testing.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -734,7 +734,7 @@ void RWSplitSession::clientReply(GWBUF* writebuf, DCB* backend_dcb)
         /** Got a complete reply, decrement expected response count */
         m_expected_responses--;
 
-        if (!backend->is_replaying_history())
+        if (!backend->is_replaying_history() && !backend->local_infile_requested())
         {
             session_book_server_response(m_pSession, backend->backend()->server, m_expected_responses == 0);
         }