Fix a bug I introduced into the code generator. :(

llvm-svn: 3694
diff --git a/llvm/lib/CodeGen/InstrSelection/InstrSelectionSupport.cpp b/llvm/lib/CodeGen/InstrSelection/InstrSelectionSupport.cpp
index 036b1a7..c651f01 100644
--- a/llvm/lib/CodeGen/InstrSelection/InstrSelectionSupport.cpp
+++ b/llvm/lib/CodeGen/InstrSelection/InstrSelectionSupport.cpp
@@ -114,7 +114,7 @@
 {
   InstructionNode* gepNode = dyn_cast<InstructionNode>(ptrNode);
   GetElementPtrInst* gepInst =
-    dyn_cast_or_null<GetElementPtrInst>(gepNode->getInstruction());
+    dyn_cast_or_null<GetElementPtrInst>(gepNode ? gepNode->getInstruction() :0);
 
   // ptr value is not computed in this tree or ptr value does not come from GEP
   // instruction