gdbstub: Fix handling of '!' packet with new infra

gdbstub: Fix handling of '!' packet with new infra

Since the '!' packet is not handled by the new infrastructure,
gdb_handle_packet() would call run_cmd_parser() with a NULL cmd_parser
value, which would lead to an unsupported packet ("$#00") being sent,
which could confuse the gdb client.

This also has a side-effect of speeding up the initial connection with
gdb.

Fixes: 3e2c12615b52 ("gdbstub: Implement deatch (D pkt) with new infra")
Signed-off-by: Ramiro Polla <ramiro.polla@gmail.com>
Message-Id: <20190805190901.14072-1-ramiro.polla@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>