Prevent null pointer access

Thanks to Ingo for fixing the segfault.

rtengine/improccoordinator.cc

@@ -364,7 +364,7 @@ void ImProcCoordinator::updatePreviewImage(int todo, bool panningRelatedChange)
             }
 
             imgsrc->getRAWHistogram(histRedRaw, histGreenRaw, histBlueRaw);
-            hist_raw_dirty = !hListener->updateHistogramRaw();
+            hist_raw_dirty = !(hListener && hListener->updateHistogramRaw());
 
             highDetailPreprocessComputed = highDetailNeeded;
 

rtengine/improccoordinator.h

@@ -469,7 +469,9 @@ public:
             hListener->setObservable(nullptr);
         }
         hListener = h;
-        h->setObservable(this);
+        if (h) {
+            h->setObservable(this);
+        }
     }
     void setAutoCamListener  (AutoCamListener* acl) override
     {