Buffering should truncate if read() returns 0 since it's not a valid return there...

Buffering should truncate if read() returns 0 since it's not a valid return there as there should be data left to read. The loop wouldn't break until there was a message in the queue. I just experienced the case with crosslinked files and read stopped making progress, returning 0 each time it was called.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@29626 a1c6a512-1295-4272-9138-f99709370657