Check avctx width/height more thoroughly (e.g. all values 0 except width would
have been accepted before).
Also do not fail if they are invalid but instead override them to 0.
This allows decoding e.g. MPEG video when only the container values are corrupted.
For encoding a value of 0,0 of course makes no sense, but was allowed
through before and will be caught by an extra check in the encode function.

Originally committed as revision 25124 to svn://svn.ffmpeg.org/ffmpeg/trunk
