xml2 locks the XML file if the file has XML syntax error

[jmendys]

Steps to reproduce

1. Create an XML file with an invalid structure for example
2. Open the file with xml2::read_xml() command

Expected behavior:

• an error is displayed
• the process is terminated

Actual behavior:

• an error is displayed
• the input XML file is locked (can't be modified or deleted) until R session is restarted (the process seems to continue in the background in a separate thread)
• if you keep calling xml2::read_xml() the R studio will eventually crash.

[hadley]

I think this is likely an interaction between libxml2 and windows that unfortunately there's nothing we can do about.

[hadley]

Actually this looks like #376