Fix crash connecting a NULL producer

[j-b-m]

Debugging a Kdenlive crash, I came upon an MLT file that consistently crashed melt.
The .mlt file was buggy, missing an XML playlist, still I think it's good if we can fix the crash in MLT.
My patch doesn't allow anymore to connect a null producer to the service. Let me know if you think there is a reason to allow that.

A sample of a crashing malformed .mlt file, the playlist1 defined at the end as a track producer is never defined :

<?xml version="1.0"?>
<mlt LC_NUMERIC="C" version="7.23.0">
  <producer id="producer0" in="0" out="14999">
    <property name="length">15000</property>
    <property name="eof">pause</property>
    <property name="resource">red</property>
    <property name="mlt_service">color</property>
  </producer>
  <playlist id="playlist0">
    <entry producer="producer0" in="0" out="14999"/>
  </playlist>
  <tractor id="tractor0" title="color:red" in="0" out="14999">
    <track producer="playlist0"/>
    <track producer="playlist1"/>
  </tractor>
</mlt>