Bug #18591 Repeated "show" and "all status" calls to ndb_mgm causes mgmd to crash

- not having lock can cause "block number list" to become corrupt => crash
- also may result in 2 threads receiving same block no => possble cause of
Bug #13987 Cluster: Loss of data nodes can cause high CPU usage from ndb_mgmd

ndb/src/mgmsrv/MgmtSrvr.cpp

@@ -2107,6 +2107,7 @@ int
 MgmtSrvr::abortBackup(Uint32 backupId)
 {
   SignalSender ss(theFacade);
+  ss.lock(); // lock will be released on exit
 
   bool next;
   NodeId nodeId = 0;

ndb/src/ndbapi/SignalSender.cpp

@@ -75,7 +75,9 @@ SignalSender::SignalSender(TransporterFacade *facade)
 {
   m_cond = NdbCondition_Create();
   theFacade = facade;
+  lock();
   m_blockNo = theFacade->open(this, execSignal, execNodeStatus);
+  unlock();
   assert(m_blockNo > 0);
 }