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.

gdb/python/lib/gdb/FrameDecorator.py

@@ -258,7 +258,10 @@ class FrameVars(object):
         are no frame local variables, return an empty list."""
         lvars = []
 
-        block = self.frame.block()
+        try:
+            block = self.frame.block()
+        except RuntimeError:
+            block = None
 
         while block != None:
             if block.is_global or block.is_static:
@@ -279,7 +282,12 @@ class FrameVars(object):
         there are no frame argument variables, return an empty list."""
 
         args = []
-        block = self.frame.block()
+
+        try:
+            block = self.frame.block()
+        except RuntimeError:
+            block = None
+
         while block != None:
             if block.function != None:
                 break