Ensure that slave->cstate contains meaningful value.

In blr_slave_callback the bits of slave->cstate are reset and
set as one transaction. Earlier they were reset in one and
set in another, leading to a situation where slave->cstate did
not contain a sensible value for a short period of time.

Further, it is now explicitly checked in blr_distribute_binlog_record
that slave->cstate indeed contains a meaningful value.

server/modules/routing/binlog/blr_master.c

@@ -1602,6 +1602,11 @@ unsigned int cstate;
 			/* Slave is in catchup mode */
 			action = 3;
 		}
+        else
+        {
+            MXS_ERROR("slave->cstate does not contain a meaningful state %d", slave->cstate);
+            action = 0;
+        }
 		slave->stats.n_actions[action-1]++;
 		spinlock_release(&slave->catch_lock);