Don't rely on inferior_ptid in record_full_wait

The multi-target patch sets inferior_ptid to null_ptid before handling
a target event, and thus before calling target_wait, in order to catch
places in target_ops::wait implementations that are incorrectly
relying on inferior_ptid (which could otherwise be a ptid of a
different target, for example).  That caught this instance in
record-full.c.

Fix it by saving the last resumed ptid, and then using it in
record_full_wait_1, just like how the last "step" argument passed to
record_full_target::resume is handled too.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* record-full.c (record_full_resume_ptid): New global.
	(record_full_target::resume): Set it.
	(record_full_wait_1): Use record_full_resume_ptid instead of
	inferior_ptid.