MXS-1503: Make sure backend is in use before using it

The get_backend_from_dcb function needs to check that the backend is in
use before comparing the pointer. This prevents stale pointers from being
used and is logically more sound than relying on raw DCB matches.

server/core/backend.cc

@@ -207,6 +207,7 @@ bool Backend::connect(MXS_SESSION* session, SessionCommandList* sescmd)
 
 bool Backend::write(GWBUF* buffer, response_type type)
 {
+    ss_dassert(in_use());
     bool rval = m_dcb->func.write(m_dcb, buffer) != 0;
 
     if (rval && type == EXPECT_RESPONSE)