hevc_mvs: initialize the temporal MV in case of missing ref

The caller expects the MV to always be initialized.
hevc_mvs: make sure to always initialize the temporal MV fully

The spec requires this.

Fixes uninitialized reads on some samples.

Remove now unnecessary initialization of the whole merge candidate list.
h263dec: Force padding bug workaround for H.263.

Fixes decoding of http://samples.mplayerhq.hu/V-codecs/h263/h263-raw/messenger.h263

Signed-off-by: Anton Khirnov <anton@khirnov.net>