Ensure possible NULL pointer values dealt with defensively

Coverity scanning shows a couple of locations where a NULL
pointer dereference could happen; updated code to ensure it
cannot.

Settings.c

@@ -543,9 +543,10 @@ static bool Settings_read(Settings* this, const char* fileName, unsigned int ini
          if (screen)
             screen->allBranchesCollapsed = atoi(option[1]);
       } else if (String_eq(option[0], ".dynamic")) {
-         if (screen)
+         if (screen) {
             free_and_xStrdup(&screen->dynamic, option[1]);
-         Platform_addDynamicScreen(screen);
+            Platform_addDynamicScreen(screen);
+         }
       }
       String_freeArray(option);
    }

pcp/Instance.c

@@ -62,10 +62,12 @@ static void Instance_writeField(const Row* super, RichString* str, RowField fiel
    const Settings* settings = super->host->settings;
    DynamicColumn* column = Hashtable_get(settings->dynamicColumns, field);
    PCPDynamicColumn* cp = (PCPDynamicColumn*) column;
-   const pmDesc* descp = Metric_desc(cp->id);
+   if (!cp)
+      return;
 
    pmAtomValue atom;
    pmAtomValue *ap = &atom;
+   const pmDesc* descp = Metric_desc(cp->id);
    if (!Metric_instance(cp->id, instid, this->offset, ap, descp->type))
       ap = NULL;