avcodec/mpeg12enc: Always initialize MPEG-2 intra VLC table lengths

avcodec/mpeg12enc: Always initialize MPEG-2 intra VLC table lengths

The MPEG-1/2 encoders initialize several tables once during the first
time one of the encoders is initialized; the table for MPEG-2 intra VLC
lengths is only initialized if it is used for this encoder instance.
This implies that if the first MPEG-1/2 encoder to be initialized does
not use it, it will never be initialized even if a later encoder
instance makes use of them. Fix this by initializing this table
unconditionally.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>