Fix for bug #10798: If relay log fails to rotate, slave will crash after

The relay log may not be open for some reason (e.g. disk error) after rotation,
and using it causes the slave crash.

Fix: check we have it open before access, return error otherwise.

sql/slave.cc

@@ -4284,6 +4284,13 @@ Log_event* next_event(RELAY_LOG_INFO* rli)
 	hot_log=0;				// Using old binary log
       }
     }
+    /* 
+      As there is no guarantee that the relay is open (for example, an I/O
+      error during a write by the slave I/O thread may have closed it), we
+      have to test it.
+    */
+    if (!my_b_inited(cur_log))
+      goto err;
 #ifndef DBUG_OFF
     {
       char llbuf1[22], llbuf2[22];