avcodec/svq3: Don't copy watermarked frame data twice

avcodec/svq3: Don't copy watermarked frame data twice

The SVQ3 decoder modifies the input bitstream at two places.
One of them is only reached when the file is watermarked.
Therefore commit 2264c1108135380c49fdf0aef97520bf77a6ed37
made a copy of all the frame data in this case.

But there is a second possibility for modifying the frame and
therefore Libav commit 1098f5c0495c61a98d4ff6b8e24c17974d4bace5
made the decoder always copy the data. This of course makes
the additional copy for watermarked frames redundant, but it hasn't
been removed. This commit does so.

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