Extra check if Process_getAffinity fails.

I could not reproduce the crash in current SVN 
(I've been adding checks for problems like this),
but the sanity check looks correct.
Thanks to Cybjit. Closes #3481053.

htop.c

@@ -772,6 +772,7 @@ int main(int argc, char** argv) {
             break;
 
          Affinity* affinity = Process_getAffinity((Process*) Panel_getSelected(panel));
+         if (!affinity) break;
          Panel* affinityPanel = AffinityPanel_new(pl, affinity);
          Affinity_delete(affinity);