Fix DCB validity checks

The DCB was always assumed to be non-NULL.

server/core/dcb.c

@@ -2503,7 +2503,7 @@ dcb_call_callback(DCB *dcb, DCB_REASON reason)
 int
 dcb_isvalid(DCB *dcb)
 {
-    return !dcb->dcb_is_zombie;
+    return dcb && !dcb->dcb_is_zombie;
 }
 
 /**

server/core/session.c

@@ -427,7 +427,7 @@ session_final_free(SESSION *session)
 int
 session_isvalid(SESSION *session)
 {
-    return true;
+    return session != NULL;
 }
 
 /**