Don't crash on null error

This happens if you try and debug a stuck process using

    gdb> print 'v8::V8::TerminateExecution'(0)

from http://remysharp.com/2013/09/11/how-i-fixed-an-anonymous-infinite-loop-in-jsbin/
Merge pull request #24 from bugsnag/null-error

Don't crash on null error