sdl/consumer_sdl_preview.c: check for null pointer

sdl/consumer_sdl_preview.c: check for null pointer

Fixes Coverity CID 709403: Dereference before null check (REVERSE_INULL)
Dereferencing pointer "this->play". [show details]
290        int eos_threshold = 20 + mlt_properties_get_int( MLT_CONSUMER_PROPERTIES( this->play ), "buffer" );
...
Assigning: "this->active" = "this->play".
424                                        this->active = this->play;
...
Dereferencing "this->play" before a null check.
466        if ( this->play ) mlt_consumer_stop( this->play );