Fix a crash in SqlAsyncTask::call caused by the fact that the SQLOperation to be called was a NULL pointer.

The crash was mostly noticable on servers with relatively high MapUpdate Threads, since the window between SqlAsyncTask::call()´s NULLpointer check for m_op and the usage of this pointer was larger when more map threads are calling Database::CommitTransaction()´s and NULLing this pointer.
Fixes crash specified in comment #24 in issue #1259, and most likely will affect other crashes specified there as well.
Courtesy of click and myself, and our lengthy and deep brainstorming session.

--HG--
branch : trunk

src/shared/Database/Database.cpp

@@ -591,7 +591,6 @@ bool Database::CommitTransaction()
     if (i != m_tranQueues.end() && i->second != NULL)
     {
         m_threadBody->Delay(i->second);
-        i->second = NULL;
         return true;
     }
     else