Fix crash when clicking on a gif image

For gif images, the dimensions is always null.

Closes #6021
//FREEBIE

src/org/thoughtcrime/securesms/components/ZoomingImageView.java

@@ -86,7 +86,7 @@ public class ZoomingImageView extends FrameLayout {
       }
 
       protected void onPostExecute(@Nullable Pair<Integer, Integer> dimensions) {
-        Log.w(TAG, "Dimensions: " + dimensions.first + ", " + dimensions.second);
+        Log.w(TAG, "Dimensions: " + (dimensions == null ? "(null)" : dimensions.first + ", " + dimensions.second));
 
         if (dimensions == null || (dimensions.first <= maxTextureSize && dimensions.second <= maxTextureSize)) {
           Log.w(TAG, "Loading in standard image view...");