[Sema] Fix crash in getConstructorName.

Summary:
Can happen when getConstructorName is called on invalid decls,
specifically the ones that do not have the injected class name.

Reviewers: bkramer, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48880

llvm-svn: 336244
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index db47931..8dd9c19 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -113,6 +113,8 @@
       break;
     }
   }
+  if (!InjectedClassName && CurClass->isInvalidDecl())
+    return ParsedType();
   assert(InjectedClassName && "couldn't find injected class name");
 
   QualType T = Context.getTypeDeclType(InjectedClassName);