fix mlt_properties_set() with invalid expression
fix mlt_properties_set() with invalid expression

Instead of setting the value as "0" this changes the behavior to treat
@whatever as a string literal instead of expression. This improves users
using a text filter getting what they expect when starting their string
with '@'.

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