glGetStringi() is not available before 3.0. We can't trust its function pointer being NULL since the GL implementation is allowed to return anything in this case (*cough* macOS's GL on Metal *cough*).
This probably regressed when fd92954df8cf12c61e08c7ef7acf172efb7c44ad removed the "if (!disabled[0]) return FALSE;" exit from filter_extensions(), although that simply exposed an existing issue.
-- v2: opengl32: Skip filter_extensions_index() on GL contexts < 3.0.