tdf#134576 at least don't crash in this edge-case

Change-Id: I69638fd2130c1ba68915906ced62bda2cd883932
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/98491
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/basic/source/runtime/runtime.cxx b/basic/source/runtime/runtime.cxx
index 607604f..9424482 100644
--- a/basic/source/runtime/runtime.cxx
+++ b/basic/source/runtime/runtime.cxx
@@ -2597,10 +2597,14 @@ void SbiRuntime::StepNEXT()
        StarBASIC::FatalError( ERRCODE_BASIC_INTERNAL_ERROR );
        return;
    }
    if( pForStk->eForType == ForType::To )
    if (pForStk->eForType != ForType::To)
        return;
    if (!pForStk->refVar)
    {
        pForStk->refVar->Compute( SbxPLUS, *pForStk->refInc );
        StarBASIC::FatalError( ERRCODE_BASIC_INTERNAL_ERROR );
        return;
    }
    pForStk->refVar->Compute( SbxPLUS, *pForStk->refInc );
}

// beginning CASE: TOS in CASE-stack