Handling of XPath function arguments in error case

The XPath engine tries to guarantee that every XPath function can pop
'nargs' non-NULL values off the stack. libxslt, for example, relies on
this assumption. But the check isn't thorough enough if there are errors
during the evaluation of arguments. This can lead to segfaults:

https://mail.gnome.org/archives/xslt/2013-December/msg00005.html

This commit makes the handling of function arguments more robust.

* Bail out early when evaluation of XPath function arguments fails.
* Make sure that there are 'nargs' arguments in the current call frame.