MXS-1323: Fix crash on attempted retry of read

When a backend is waiting for a response but no statement is stored for
the session, the buffer where the stored statement is copied is not
modified. This means that it needs to be initialized to a NULL value.

Added a test that checks that the behavior works as expected even with
persistent connections. A second test reproduces the crash by executing
parallel SET commands while slaves are blocked.

There is still a behavioral problem in readwritesplit. If a session
command is being executed and it fails on a slave, an error is sent to the
client. In this case it would not be necessary to close the session if the
master is still alive.

maxscale-system-test/mariadb_nodes.cpp

@@ -14,6 +14,7 @@
 #include "sql_const.h"
 #include <climits>
 #include <string>
+#include <sstream>
 #include <vector>
 
 Mariadb_nodes::Mariadb_nodes(const char *pref, const char *test_cwd, bool verbose):
@@ -918,6 +919,13 @@ int Mariadb_nodes::get_server_id(int index)
     return id;
 }
 
+std::string Mariadb_nodes::get_server_id_str(int index)
+{
+    std::stringstream ss;
+    ss << get_server_id(index);
+    return ss.str();
+}
+
 void Mariadb_nodes::generate_ssh_cmd(char *cmd, int node, const char *ssh, bool sudo)
 {
     if (strcmp(IP[node], "127.0.0.1") == 0)