[Sema] Fix a structured binding typo correction bug

BindingDecls have null type until their initializer is processed, so we can't
assume that a correction candidate has non-null type.

rdar://41559582

llvm-svn: 336634
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 984247b..4545bf9 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -4990,6 +4990,8 @@
         // determine if it is a pointer or reference to a function. If so,
         // check against the number of arguments expected for the pointee.
         QualType ValType = cast<ValueDecl>(ND)->getType();
+        if (ValType.isNull())
+          continue;
         if (ValType->isAnyPointerType() || ValType->isReferenceType())
           ValType = ValType->getPointeeType();
         if (const FunctionProtoType *FPT = ValType->getAs<FunctionProtoType>())