Don't use context when it's null in XML reporter

Unfortunately there's no test to back this up. Extensive attempts at reproducing
has been made without luck. It seems that every once in a while, one context too
many is popped, and chaos ensues. However, when this happens, the report is
complete and fine, so aborting _should_ be safe (famous last words).

Here's a case where it's happened in the past: https://gist.github.com/magnars/444b5ec840ae60edd9d8

This fixes #263