Fix crash when using archaic protocol, rdar://10238337

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@141215 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/SelectorLocationsKind.cpp b/lib/AST/SelectorLocationsKind.cpp
index 9a44b38..671207a 100644
--- a/lib/AST/SelectorLocationsKind.cpp
+++ b/lib/AST/SelectorLocationsKind.cpp
@@ -54,8 +54,11 @@
 
 template <>
 SourceLocation getArgLoc<ParmVarDecl>(ParmVarDecl *Arg) {
+  SourceLocation Loc = Arg->getLocStart();
+  if (Loc.isInvalid())
+    return Loc;
   // -1 to point to left paren of the method parameter's type.
-  return Arg->getLocStart().getLocWithOffset(-1);
+  return Loc.getLocWithOffset(-1);
 }
 
 template <typename T>