mlt_consumer_start(): check return value from mlt_properties_get_int()

mlt_consumer_start(): check return value from mlt_properties_get_int()

Fixes Coverity CID 709343: Division or modulo by zero (DIVIDE_BY_ZERO)
Division by expression "mlt_properties_get_int(properties, "frame_rate_num")" which may be zero has undefined behavior
On this path, function call "mlt_properties_get_int(properties, "frame_rate_num")" has return value of 0
 442        int frame_duration = 1000000 / mlt_properties_get_int( properties, "frame_rate_num" ) *
 443                        mlt_properties_get_int( properties, "frame_rate_den" );