gdbserver: check for nullptr condition in regcache::get_register_status

gdbserver: check for nullptr condition in regcache::get_register_status

A regcache can be initialized with a register value buffer, in which
case, the register_status pointer is null.  This condition is checked
in set_register_status, but not in get_register_status.  Do this check
for consistence and safety.

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