lib/gis: Close va_list if erroring out early in debug module

[ymdatta]

Even though, the defined va_list will not be used anywhere, it's a good practice to close it as not doing so leads to undefined behavior.

This was found using cppcheck tool.

[metzm]

Start ap as late as possible and close it as early as possible ?

        va_start(ap, msg);
        vfprintf(fd, msg, ap);
        va_end(ap);

i.e. move va_start() and va_end() to be directly around vfprintf()

See also error.c