[Sema] Avoid crash for category implementation without interface

[smeenai]

Summary

When we have a category implementation without a corresponding interface
(which is an error by itself), semantic checks for property accesses
will attempt to access a null interface declaration and then segfault.
Error out in such cases instead.

I'm not sure if it's best to add the null check where I've added it or
if it should be fixed further up the stack.

[rjmccall]

I think that's a fine place to add the check.

lib/Sema/SemaExprMember.cpp

if (OT->isObjCClass()) {
                                   ObjCImpDecl, HasTemplateArgs);
 
         goto fail;
       }
 
       // Also must look for a getter name which uses property syntax.
       Selector Sel = S.PP.getSelectorTable().getNullarySelector(Member);
       ObjCInterfaceDecl *IFace = MD->getClassInterface();
+      if (!IFace)
+        goto fail;
+
       ObjCMethodDecl *Getter;
       if ((Getter = IFace->lookupClassMethod(Sel))) {
         // Check the use of this method.
         if (S.DiagnoseUseOfDecl(Getter, MemberLoc))
           return ExprError();
       } else
         Getter = IFace->lookupPrivateMethod(Sel, false);
       // If we found a getter then this may be a valid dot-reference, we

test/SemaObjC/undef-class-property-error.m

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+@implementation I (C) // expected-error {{cannot find interface declaration for 'I'}}
+
++ (void)f {
+  self.m; // expected-error {{member reference base type 'Class' is not a structure or union}}
+}
+
+@end