kdenlive/producer_framebuffer.c: check return values

kdenlive/producer_framebuffer.c: check return values

Check calloc() return and fix Coverity CID 709328: Unchecked return value (CHECKED_RETURN)
Calling function "mlt_producer_init" without checking return value (as is done elsewhere 19 out of 20 times).
No check of the return value of "mlt_producer_init(producer, NULL)".
254        mlt_producer_init( producer, NULL );