starmath: fix -Werror=unused-variable

Seen with gcc 7.5.0.

Change-Id: I34940865f62ff397a3a6813fbabd0fb45a01d88f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/155654
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins
diff --git a/starmath/source/symbol.cxx b/starmath/source/symbol.cxx
index 1523543..3baf797 100644
--- a/starmath/source/symbol.cxx
+++ b/starmath/source/symbol.cxx
@@ -117,8 +117,9 @@ SmSym *SmSymbolManager::GetSymbolByName(const OUString& rSymbolName)
        pRes = &aIt->second;
    else
    {
        for (auto& [_, rSymbol] : m_aSymbols)
        for (auto& rPair : m_aSymbols)
        {
            SmSym& rSymbol = rPair.second;
            if (rSymbol.GetExportName() == rSymbolName)
            {
                pRes = &rSymbol;