Fix crash when encoding using libschroedinger.

Currently only pixel and half-pixel motion vector precisions are supported in
libschroedinger. Setting the mv_precision field to 2 (i.e. quarter pixel)
causes a crash in the libschroedinger encoder calls. By not setting this
parameter, we fall back to the default value used in libschroedinger.
patch by Anuradha Suraparaju, anuradha rd.bbc.co uk

Originally committed as revision 17132 to svn://svn.ffmpeg.org/ffmpeg/trunk