firebird: return consistent (if wrong...) results for XMultipleResults

Just in case we are called by code not checking our DatabaseMetadata
to learn we don't support multiple resultsets.

Change-Id: I6d8c5f4ef04bcd9ec8b66a83881607281e125b7b
Reviewed-on: https://gerrit.libreoffice.org/47117Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>

connectivity/source/drivers/firebird/StatementCommonBase.cxx

@@ -241,7 +241,7 @@ sal_Bool SAL_CALL OStatementCommonBase::getMoreResults()
 sal_Int32 SAL_CALL OStatementCommonBase::getUpdateCount()
 {
     // TODO: verify we really can't support this
-    return 0;
+    return -1;
 }