disas/nanomips: Prevent memory leaking

disas/nanomips: Prevent memory leaking

g_autofree attribute is added for every dynamically allocated string to
prevent memory leaking.

The implementation of the several functions that work with dynamically
allocated strings is slightly changed so we can add those attributes.

Signed-off-by: Milica Lazarevic <milica.lazarevic@syrmia.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220912122635.74032-19-milica.lazarevic@syrmia.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>