InsetIndex: make a condition more bullet-proof, a nullptr could be dereferenced.
Error noticed by Coverity:
*** CID 382777: Memory - illegal accesses (RETURN_LOCAL)
/home/lasgoutt/src/lyx/coverity/lyx/src/insets/InsetIndex.cpp: 1866 in _ZNK3lyx15InsetPrintIndex5xhtmlB5cxx11ERNS_9XMLStreamERKNS_12OutputParamsE()
1860
1861 // Collect the index entries in a form we can use them.
1862 vector<IndexEntry> entries;
1863 const docstring & indexType = params().getParamOr("type", from_ascii("idx"));
1864 for (const TocItem& item : *toc) {
1865 const auto* inset = static_cast<const InsetIndex*>(&(item.dit().inset()));
>>> CID 382777: Memory - illegal accesses (RETURN_LOCAL)
>>> Using "indexType", which points to an out-of-scope temporary variable of type "lyx::docstring const".
1866 if (item.isOutput() && inset->params().index == indexType)
1867 entries.emplace_back(IndexEntry{inset, &op});
1868 }
1869
1870 // If all the index entries are in notes or not displayed, get out sooner.
1871 if (entries.empty())