Fix crash if running property was never set.
Fix crash if running property was never set.

Reported by Victordmdb@github in https://github.com/mltframework/mlt/
issues/352#issuecomment-410456480

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