displaced_step_finish: Don't fetch the regcache of exited threads

displaced_step_finish: Don't fetch the regcache of exited threads

displaced_step_finish can be called with event_status.kind ==
TARGET_WAITKIND_THREAD_EXITED, and in that case it is not possible to
get at the already-exited thread's registers.

This patch moves the get_thread_regcache calls to branches that
actually need it, where we know the thread is still alive.

It also adds an assertion to get_thread_regcache, to help catching
these broken cases sooner.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I63b5eacb3e02a538fc5087c270d8025adfda88c3