starmath: Replace OSL_ENSURE with assert

as negative n would be undefined for OUString::copy(..., n).

Change-Id: I950ec60656142450a5d5f6804324f6080c9c4c2a
Reviewed-on: https://gerrit.libreoffice.org/26753
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

starmath/source/parse.cxx

@@ -460,7 +460,7 @@ void SmParser::NextToken()
             --aRes.EndPos;
         }
         sal_Int32 n = aRes.EndPos - nRealStart;
-        OSL_ENSURE( n >= 0, "length < 0" );
+        assert(n >= 0);
         m_aCurToken.eType      = TNUMBER;
         m_aCurToken.cMathChar  = '\0';
         m_aCurToken.nGroup     = TG::NONE;
@@ -482,7 +482,7 @@ void SmParser::NextToken()
     else if (aRes.TokenType & KParseType::IDENTNAME)
     {
         sal_Int32 n = aRes.EndPos - nRealStart;
-        OSL_ENSURE( n >= 0, "length < 0" );
+        assert(n >= 0);
         OUString aName( m_aBufferString.copy( nRealStart, n ) );
         const SmTokenTableEntry *pEntry = GetTokenTableEntry( aName );