Fix check for bound methods on falsy objects.

[Yorwba]

Minimal reproduction of the bug fixed by this PR:

class Falsy(object):
	def __bool__(self):
		return False
	__nonzero__ = __bool__
	def do_stuff(self):
		pass

Falsy().do_stuff()

ERROR:byterun.pyvm2:Caught exception during execution
Traceback (most recent call last):
  File "byterun/pyvm2.py", line 236, in dispatch
    why = bytecode_fn(*arguments)
  File "byterun/pyvm2.py", line 925, in byte_CALL_FUNCTION
    return self.call_function(arg, [], {})
  File "byterun/pyvm2.py", line 956, in call_function
    if not isinstance(posargs[0], func.im_class):
IndexError: list index out of range

Because instances of Falsy are falsy, if func.im_self treats them as nonexistent and does not add the self parameter to the argument list.

[rocky]

@Yorwba @andychu - Thanks - I've applied this, including the test on the fork of this project (now called x-python)

I should note that the code given appears to be a problem in Python 2.x, not 3.x.