[analyzer] Fix a bug in RetainReleaseChecker diagnostics. It gives more precise error message on the modified test case (and prevents duplicate diagnostics when we purge at block granularity).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140840 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/StaticAnalyzer/Checkers/RetainCountChecker.cpp b/lib/StaticAnalyzer/Checkers/RetainCountChecker.cpp
index 7791d9d..695cc03 100644
--- a/lib/StaticAnalyzer/Checkers/RetainCountChecker.cpp
+++ b/lib/StaticAnalyzer/Checkers/RetainCountChecker.cpp
@@ -3436,7 +3436,7 @@
   const ProgramState *state = Builder.getState();
   GenericNodeBuilderRefCount Bd(Builder);
   RefBindings B = state->get<RefBindings>();
-  ExplodedNode *Pred = 0;
+  ExplodedNode *Pred = Builder.getPredecessor();
 
   for (RefBindings::iterator I = B.begin(), E = B.end(); I != E; ++I) {
     llvm::tie(Pred, state) = handleAutoreleaseCounts(state, Bd, Pred, Eng,