DECODE_STATE_STOP is always set as dc->state, and dc->stop
is always cleared.  So handle it in decodeStart once rather
than doing it in every plugin.

While we're at it, fix a long-standing (but difficult to
trigger) bug in mpc_decode where we failed to return
if mpc_decoder_initialize() fails.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7122 09075e82-0dd4-0310-85a5-a0d7c8717e4f