gdbserver: Fix incorrect assertion

While playing with adding a new event kind, I noticed that
prepare_resume_reply TARGET_WAITKIND_FORKED, etc. advance 'buf', so if
we force-disable the T packet, we'd fail the *buf == 'T' assertion.

Fix it by tweaking the assertion to always look at the beginning of
the buffer.

Change-Id: I8c38e32353db115edcde418b3b1e8ba12343c22b