MXS-2450: Don't discard history if it's disabled

If the session command history is not enabled, it shouldn't be discarded
when a COM_CHANGE_USER is executed.

maxscale-system-test/mxs2450_change_user_crash.cpp

@@ -0,0 +1,20 @@
+/**
+ * MXS-2450: Crash on COM_CHANGE_USER with disable_sescmd_history=true
+ * https://jira.mariadb.org/browse/MXS-2450
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char *argv[])
+{
+    TestConnections test(argc, argv);
+    Connection conn = test.maxscales->rwsplit();
+    test.expect(conn.connect(), "Connection failed: %s", conn.error());
+
+    for (int i = 0; i < 10; i++)
+    {
+        test.expect(conn.reset_connection(), "Connection reset failed: %s", conn.error());
+    }
+
+    return test.global_result;
+}