Move the inner loop which converts samples to flac_convert().  There
it is isolated and easier to optimize.  This function does not have to
worry about buffer boundaries; the caller (i.e. flacWrite())
calculates how much is left and is responsible for flushing.  That
saves a lot of superfluous range checks within the loop.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7327 09075e82-0dd4-0310-85a5-a0d7c8717e4f