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.
fix erroneous error-handling in frame filter code

This fixes PR python/16487.

The bug here is that the function-name-handling code in py_print_frame
had a small logic error (really a misplaced closing brace).  This
error could lead to a Py_DECREF(NULL), which crashes.

This patch fixes the bug in the obvious way.

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

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

	PR python/16487:
	* python/py-framefilter.c (py_print_frame): Don't call Py_DECREF
	on a NULL pointer.  Move "goto error" to correct place.

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

	PR python/16487:
	* gdb.python/py-framefilter.exp: Add test using "Error" filter.
	* gdb.python/py-framefilter.py (ErrorInName, ErrorFilter): New
	classes.
avoid python exception in FrameDecorator.py

This fixes a bug in FrameDecorator.py.

FrameVars seems to assume that Frame.block can return None if there is
no block.  However, it actually throws an exception.

I saw this bug while developing a frame filter, but unfortunately I
don't know how to reproduce it.  It seems to me that the SAL tests in
_is_limited_frame should exclude the bad cases; and in my attempts to
write a test they do.

Nevertheless I think the fix is reasonably obvious and ought to go in.

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

	PR python/16485:
	* python/lib/gdb/FrameDecorator.py: (FrameVars.fetch_frame_args):
	Handle exception from frame.block.
	(FrameVars.fetch_frame_locals): Likewise.