gdbstub: clean-up vcont handling to avoid goto

gdbstub: clean-up vcont handling to avoid goto

We can handle all the error exit cases by using g_autofree() for the
one thing that needs cleaning up on the exit.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-31-alex.bennee@linaro.org>