[Bug] Emacs frozen in python mode with which-func & narrow

[Levin]

Start emacs with 'emacs -Q', open a python file with content below:
--8<--8<--
#This is a comment

def f():
    print "Hello world"
--8<--8<--

Then narrow region to avoid display of comment:
--8<--8<--

def f():
    print "Hello world"
--8<--8<--

OK, now `M-<', `M-x which-func-mode', `M-x indent-relative'.

Then emacs will froze. `C-g' has no effect.

--
Levin