avcodec/avcodec: Use avcodec_close() on avcodec_open2() failure

avcodec/avcodec: Use avcodec_close() on avcodec_open2() failure

Compared to the earlier behaviour the following changes:
a) AVCodecInternal.byte_buffer is freed.
b) The last_pkt_props FIFO is emptied before freeing it.
c) If set AVCodecContext.hwaccel is uninitialized and its private data
is freed; hw_frames_ctx and hw_device_ctx are also unreferenced.
d) coded_side_data is freed.
e) active_thread_type is reset.
a), b), d) should be no-ops as the buffer/fifo should be empty and
no coded_side_data should exist at any point of avcodec_open2().
e) is obviously not bad.
c) is in accordance with the documentation of hw_(frames|device)_ctx
which states that libacodec takes over ownership of these references.
At least in the case of VC-1 it is possible for the hw acceleration to
be set during init and in this case freeing it actually fixes a memleak.

avcodec_close() needed only minor adjustments to make it work with
a potentially not fully initialized codec.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>