starmath: This condition must be met

Otherwise, the following static_cast would be broken.

Change-Id: Iad4738292ed6903eed78d5af98f77194d5a1d8fe
Reviewed-on: https://gerrit.libreoffice.org/26696
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Takeshi Abe <tabe@fixedpoint.jp>
diff --git a/starmath/source/parse.cxx b/starmath/source/parse.cxx
index 7d02bad..b7a0cf0 100644
--- a/starmath/source/parse.cxx
+++ b/starmath/source/parse.cxx
@@ -1508,10 +1508,7 @@ void SmParser::DoTerm(bool bGroupNumberIdent)
                        DoFontAttribut();

                    SmNode* pTmp = popOrZero(m_aNodeStack);

                    // check if casting in following line is ok
                    OSL_ENSURE(pTmp && !pTmp->IsVisible(), "Sm : Ooops...");

                    assert(pTmp && !pTmp->IsVisible());
                    aStack.push(static_cast<SmStructureNode *>(pTmp));
                }