Fixed a crash reporting errors from a CBLReplication

NOTE: This patch was made on the stable branch as a fix for beta 2.
It does _not_ need to be merged into master, because CBLReplicator has
been rewritten since beta 2 and this bug doesn’t exist there.

Fixes #220

Source/API/CBLReplication.m

@@ -568,11 +568,14 @@ - (void) bg_updateProgress: (CBL_Replicator*)tdReplicator {
         mode = tdReplicator.active ? kCBLReplicationActive : kCBLReplicationIdle;
 
     // Communicate its state back to the main thread:
+    NSError* error = tdReplicator.error;
+    NSUInteger changesProcessed = tdReplicator.changesProcessed;
+    NSUInteger changesTotal = tdReplicator.changesTotal;
     MYOnThread(_mainThread, ^{
         [self updateMode: mode
-                   error: tdReplicator.error
-               processed: tdReplicator.changesProcessed
-                 ofTotal: tdReplicator.changesTotal];
+                   error: error
+               processed: changesProcessed
+                 ofTotal: changesTotal];
     });
 
     if (_bg_replicator && mode == kCBLReplicationStopped) {