Do not free the header while unmapping the xxxx_UNPREPARE messages

[Eric Pouech]

> Freeing the header while unmapping the 16/32-bit message is an alien task
> for mapping functions IMO.

since the buffer (the 16 bit segmented to be used instead of the 32bit 
linear) has been allocated in the mapping function, I do think it's a 
bad idea to free it outside of the unmapping function

A+
-- 
Eric Pouech