Initialize libxml2 global memory in the main()

There could happen crashes in libxml2 when being used for the first
time in multiple threads at once, like with xmlFindCharEncodingHandler()
function. This and some others require initialization in the main thread.

One example of such crash can be found in a downstream bug report:
https://bugzilla.redhat.com/show_bug.cgi?id=1380268