byterun: Fix check for bound methods on falsy objects. (#95)

The test failures in core/glob_test, core/lexer_gen_test and core/lex_test were caused by calling a method on a falsy object and byterun incorrectly treating the `self` parameter as nonexistent.

opy/byterun/pyvm2.py

@@ -872,7 +872,7 @@ def call_function(self, arg, args, kwargs):
            #debug('im_self %r', (func.im_self,))
            #debug('posargs %r', (posargs,))

            if func.im_self:
            if func.im_self is not None:
                posargs.insert(0, func.im_self)

            #debug('posargs AFTER %r', (posargs,))