watch_command_1: Fix dangling frame access

watch_command_1: Fix dangling frame access

While working on some changes to switch_to_thread, I inadvertently
make switch_to_thread call reinit_frame_cache more frequently, even
when the thread didn't change.  This exposed a latent bug in
watch_command_1, where we're referencing a frame after
creating/inserting breakpoints, which potentially calls
reinit_frame_cache if it needs to install breakpoints with a different
thread selected.

Handle this similarly to how it's already handled in other similar
cases.  I.e., save any frame-related information we might need before
creating a breakpoint.

gdb/ChangeLog:
2017-04-13  Pedro Alves  <palves@redhat.com>

	* breakpoint.c (watch_command_1): Save watchpoint-frame info
	before calling create_internal_breakpoint.