In some cases, the "fail BB dominator" may be null after the BB was split (and

becomes reachable when before it wasn't). Check to make sure that it's not null
before trying to use it.

llvm-svn: 128434

llvm/lib/CodeGen/StackProtector.cpp

@@ -221,7 +221,8 @@ bool StackProtector::InsertStackProtectors() {
     BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");
     if (DT && DT->isReachableFromEntry(BB)) {
       DT->addNewBlock(NewBB, BB);
-      FailBBDom = DT->findNearestCommonDominator(FailBBDom, BB);
+      if (FailBBDom)
+        FailBBDom = DT->findNearestCommonDominator(FailBBDom, BB);
     }
 
     // Remove default branch instruction to the new BB.