videostab/filter_videostab*.c: check return value from mlt_filter_new()

videostab/filter_videostab*.c: check return value from mlt_filter_new()

Fixes Coverity CID 709365 and 709366:
Dereference null return value (NULL_RETURNS)
Function "mlt_filter_new" returns null (checked 50 out of 52 times). [show details]
Assigning: "parent" = null return value from "mlt_filter_new".
201                mlt_filter parent = mlt_filter_new();
Dereferencing a null pointer "parent".
202                parent->child = self;