mlt_consumer_new(): handle return value from mlt_consumer_init()

mlt_consumer_new(): handle return value from mlt_consumer_init()

If init() fails return NULL. Fixes Coverity CID 709325.

At conditional (1): "self != NULL" taking the true branch.
 339        if ( self != NULL )
CID 709325: Unchecked return value (CHECKED_RETURN)
Calling function "mlt_consumer_init" without checking return value (as is done elsewhere 10 out of 11 times).
No check of the return value of "mlt_consumer_init(self, NULL, profile)".
 340                mlt_consumer_init( self, NULL, profile );