Fixes crash (since ee07490) on loading empty MessagesVC

//FREEBIE

Signal/src/view controllers/MessagesViewController.m

@@ -155,9 +155,12 @@ typedef enum : NSUInteger {
 {
     [super viewWillAppear:animated];
     
-    NSIndexPath * lastCellIndexPath = [NSIndexPath indexPathForRow:(NSInteger)[self.messageMappings numberOfItemsInGroup:self.thread.uniqueId]-1 inSection:0];
-    [self.collectionView scrollToItemAtIndexPath:lastCellIndexPath atScrollPosition:UICollectionViewScrollPositionBottom animated:NO];
+    NSInteger numberOfMessages = (NSInteger)[self.messageMappings numberOfItemsInGroup:self.thread.uniqueId];
     
+    if (numberOfMessages > 0) {
+        NSIndexPath * lastCellIndexPath = [NSIndexPath indexPathForRow:numberOfMessages-1 inSection:0];
+        [self.collectionView scrollToItemAtIndexPath:lastCellIndexPath atScrollPosition:UICollectionViewScrollPositionBottom animated:NO];
+    }
 }
 
 - (void)startReadTimer{
@@ -1143,7 +1146,6 @@ typedef enum : NSUInteger {
         
         self.thread = gThread;
     }];
-
 }
 
 - (IBAction)unwindGroupUpdated:(UIStoryboardSegue *)segue{