Fix Python gdb.Breakpoint.location crash

I noticed today that gdb.Breakpoint.location will crash when applied
to a catchpoint made with "catch throw".

The bug is that "catch throw" makes a breakpoint that is of type
bp_breakpoint, but which does not have a location.

Regression tested on x86-64 Fedora 28.

gdb/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	* python/py-breakpoint.c (bppy_get_location): Handle a
	bp_breakpoint without a location.

gdb/testsuite/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	* gdb.python/py-breakpoint.exp (check_last_event): Check location
	of a "throw" catchpoint.