producer_pango_init(): check for null pointer

producer_pango_init(): check for null pointer

Fixes Coverity CID 710868: Dereference before null check (REVERSE_INULL)
Dereferencing pointer "markup". (The dereference is assumed on the basis of the 'nonnull' parameter attribute.)
219                                                strcat( markup, line );
...
Dereferencing pointer "markup". (The dereference is assumed on the basis of the 'nonnull' parameter attribute.)
228                                if ( markup[ strlen( markup ) - 1 ] == '\n' )
229                                        markup[ strlen( markup ) - 1 ] = '\0';
...
Dereferencing "markup" before a null check.
232                                mlt_properties_set( properties, "markup", ( markup == NULL ? "" : markup ) );