Avoid harmless Coverity warning in oc_sb_create_plane_mapping().

Coverity thinks we're overrunning this static array by passing it to memset.
In reality, it's a multidimensional array, and guaranteed to be contiguous, but
 we might as well silence this warning.
Reported in https://bugzilla.mozilla.org/show_bug.cgi?id=559343

svn path=/trunk/theora/; revision=17505