Fix possible exception leak in python.c

Fix possible exception leak in python.c

In the Python code, gdb exceptions may not leak into the Python core.
execute_gdb_command was calling bpstat_do_actions outside of a
TRY/CATCH; which seemed risky.  I don't have a test case for this, but
if bpstat_do_actions could ever throw, it could crash gdb.

This patch introduces a new scope in order to preserve the current
semantics, so it is looks a bit bigger than it really is.

Tested on x86-64 Fedora 28.

gdb/ChangeLog
2018-09-07  Tom Tromey  <tom@tromey.com>

	* python/python.c (execute_gdb_command): Call bpstat_do_actions
	inside the TRY.