Check for null ObjCInterfaceDecls returned from getClassInterface() when generating USRs. While I have no test case for this (could not create one), this shows up in crash reports. Tentatively fixes <rdar://problem/8452791>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@114392 91177308-0d34-0410-b5e6-96231b3b80d8

tools/libclang/CIndexUSRs.cpp

@@ -286,10 +286,17 @@ void USRGenerator::VisitObjCMethodDecl(ObjCMethodDecl *D) {
   do {
     if (ObjCCategoryDecl *CD = dyn_cast<ObjCCategoryDecl>(container))
       if (CD->IsClassExtension()) {
-        Visit(CD->getClassInterface());
-        break;
-      }    
-    Visit(cast<Decl>(D->getDeclContext()));
+        // ID can be null with invalid code.
+        if (ObjCInterfaceDecl *ID = CD->getClassInterface()) {
+          Visit(ID);
+	  break;
+        }
+        // Invalid code.  Can't generate USR.
+        IgnoreResults = true;
+        return;
+      }
+
+    Visit(container);
   }
   while (false);