fix crash in frame filters

fix crash in frame filters

apply_frame_filter calls ensure_python_env before computing the
gdbarch to use.  This means that python_gdbarch can be NULL while in
Python code, and if a frame filter depends on this somehow (easy to
do), gdb will crash.

The fix is to compute the gdbarch first.

Built and regtested on x86-64 Fedora 18.
New test case included.

2014-01-23  Tom Tromey  <tromey@redhat.com>

	PR python/16491:
	* python/py-framefilter.c (apply_frame_filter): Call
	ensure_python_env after computing gdbarch.

2014-01-23  Tom Tromey  <tromey@redhat.com>

	PR python/16491:
	* gdb.python/py-framefilter.py (Reverse_Function.function): Read a
	string from an inferior frame.
	* gdb.python/py-framefilter-mi.exp: Update.