Fix segfault when invoking custom XPath function without arguments

[waj]

Currently this produces a segmentation fault:

x = Nokogiri::XML('<root/>')
ctx = Class.new {
  def foo(x)
    "FOO"
  end
}.new

x.xpath('foo(x)', ctx)

The problem seems to be in ruby_funcall function that tries to initalize the arguments without checking.