mp4: Check return value of membuffer_transfer_from_file().

mp4: Check return value of membuffer_transfer_from_file().

This function calls the ->read() method of the callback, which may
fail. Currently all three callers ignore the return value and rely
on the fact that the membuffer is set to error state, which will be
detected later.

It's easier and clearer to check for errors in the callers and fail
early on read errors. Since the membuffer is useless in the error
case, free it right away in membuffer_transfer_from_file(). Change
the function to return bool instead of unsigned while at it and remove
a pointless cast in one of its callers.