fix a possible crash in mlt_playlist get_frame
fix a possible crash in mlt_playlist get_frame

After it fetches a producer from its playlist, this producer might have
been removed and closed by another thread while trying to get the frame
from the producer. mlt_producer should take a reference on this producer
while using it.

mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt