Do not memcpy raw video frames when using null muxer

Commit 035af99 made avconv always call an encoder when using the
null muxer.  While useful for 2-pass encodes, it inadvertently
caused an extra memcpy of raw frames when decoding only.

This hack restores the old behaviour when only decoding while
allowing use of the null muxer with encoded streams as well.

Signed-off-by: Mans Rullgard <mans@mansr.com>
