cpu->gdb_regs may be NULL if no coprocessor is registered.

Fixes: 73c392c2 ("gdbstub: Replace gdb_regs with an array")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20231029145033.592566-17-alex.bennee@linaro.org>
Message-Id: <20231019101030.128431-2-akihiko.odaki@daynix.com>
Tested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>