Keep only the last error.  This fixes bogus aborts due to the error
check in decoder_get_virtual_command().

decoder_load_replay_gain() will create the InputStream anyway, so
we're not saving any overhead by opening the InputStream on demand
only.

Give MPD clients access to the error condition.

Fixes memory leak after InputStream::Check() failure.

Remove that surprising rule.

The state ERROR can only be entered during decoder initialization, and
Seek() may only be called after initialization has been finished.

Fixes http://bugs.musicpd.org/view.php?id=4432

Allocate the buffer dynamically using av_malloc(), and free
AVIOContext.buffer in the destructor, as mandated by the libavformat
documentation.

Fixes http://bugs.musicpd.org/view.php?id=4446

Allocate the frame pointers only when actually decoding the file.

Eliminates some duplicate code, and as a side effect, this works
around clang 3.8 compiler warning because a negative value was
shifted.