Clear last transaction builder before early exit

Summary: Clean up lastTransationBuilder

Reviewed By: maxolls

Differential Revision: D27657431

fbshipit-source-id: a29e4236c60fd8a3657a457d1affbe22c55fdd37

Source/IGListKit/IGListAdapterUpdater.m

@@ -94,6 +94,7 @@
 
     if (!transaction) {
         // If we don't have enough information, we might not be able to create a transaction.
+        self.lastTransactionBuilder = nil;
         return;
     }