Fixed CORE-2173: Server crash after ubnormal disconnect with open Exe…

…cuteStatement

src/jrd/execute_statement.cpp

@@ -89,7 +89,8 @@ void startCallback(thread_db* tdbb)
 #if (defined DEV_BUILD && !defined MULTI_THREAD)
 	tdbb->getDatabase()->dbb_flags |= DBB_exec_statement;
 #endif
-	tdbb->getTransaction()->tra_callback_count++;
+	if (tdbb->getTransaction())
+		tdbb->getTransaction()->tra_callback_count++;
 	THREAD_EXIT();
 }
 
@@ -99,7 +100,8 @@ void finishCallback(thread_db* tdbb)
 #if (defined DEV_BUILD && !defined MULTI_THREAD)
 	tdbb->getDatabase()->dbb_flags &= ~DBB_exec_statement;
 #endif
-	tdbb->getTransaction()->tra_callback_count--;
+	if (tdbb->getTransaction())
+		tdbb->getTransaction()->tra_callback_count--;
 }
 
 } // anonymous namespace
@@ -294,15 +296,21 @@ void ExecuteStatement::Close(thread_db* tdbb)
 
 		Statement = 0;
 	}
-	char* p = reinterpret_cast<char*>(Sqlda);
-	delete[] p;
+
+	// if transaction is already missing, it's pool is also deleted,
+	// i.e. impossible and no use deleting something from it
+	if (tdbb->getTransaction()) {
+		char* p = reinterpret_cast<char*>(Sqlda);
+		delete[] p;
+		delete[] Buffer;
+	}
 	Sqlda = 0;
+	Buffer = 0;
+
 	if (Transaction) {
 		delete YValve::translate<YValve::Transaction>(&Transaction);
 		Transaction = 0;
 	}
-	delete[] Buffer;
-	Buffer = 0;
 }
 
 XSQLDA* ExecuteStatement::MakeSqlda(thread_db* tdbb, short n)