videostab/filter_videostab*.c: check for null

videostab/filter_videostab*.c: check for null

Fixes Coverity CID 709404:
Dereference before null check (REVERSE_INULL)
Dereferencing pointer "g". [show details]
 85        if ( !mlt_geometry_parse( g, vectors, length, -1, -1 ) )
...
Dereferencing "g" before a null check.
104        if ( g ) mlt_geometry_close( g );