gdb: add invalidate_selected_frame function

Instead of using `select_frame (nullptr)` to invalidate the selected
frame, introduce a function to do that.  There is no change in behavior,
but it makes the intent a bit clearer.  It also allows adding an assert
in select_frame that fi is not nullptr, so it avoids passing nullptr by
mistake.

Change-Id: I61643f46bc8eca428334513ebdaadab63997bdd0
Reviewed-By: Bruno Larsen <blarsen@redhat.com>