XPath: Workaround Coverity false positive
XPath: Workaround Coverity false positive

Coverity hits a similar false positive to what clang static analyzer hit
- it assumes that since optimize() checks _right for being nullptr,
optimize_self() might hit _right=nullptr in the ast_op_equal case which
is impossible.

Contributes to #236.

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml