[ODRHash] Fix null pointer dereference for ObjC selectors with empty slots.

`Selector::getIdentifierInfoForSlot` returns NULL if a slot has no
corresponding identifier. Add a boolean to the hash and a NULL check.

rdar://problem/51615164

Reviewers: rtrieu

Reviewed By: rtrieu

Subscribers: dexonsmith, cfe-commits, jkorous

Differential Revision: https://reviews.llvm.org/D63789

llvm-svn: 364664
diff --git a/clang/lib/AST/ODRHash.cpp b/clang/lib/AST/ODRHash.cpp
index e471540..3b89c63 100644
--- a/clang/lib/AST/ODRHash.cpp
+++ b/clang/lib/AST/ODRHash.cpp
@@ -71,8 +71,13 @@
     AddBoolean(S.isKeywordSelector());
     AddBoolean(S.isUnarySelector());
     unsigned NumArgs = S.getNumArgs();
+    ID.AddInteger(NumArgs);
     for (unsigned i = 0; i < NumArgs; ++i) {
-      AddIdentifierInfo(S.getIdentifierInfoForSlot(i));
+      const IdentifierInfo *II = S.getIdentifierInfoForSlot(i);
+      AddBoolean(II);
+      if (II) {
+        AddIdentifierInfo(II);
+      }
     }
     break;
   }