[Sema] Fix crash-on-invalid-code issue:

@class I;
@implementation I(cat) // crashes here
@end

rdar://10968158

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151931 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaDeclObjC.cpp b/lib/Sema/SemaDeclObjC.cpp
index 9b49260..4df5ab3 100644
--- a/lib/Sema/SemaDeclObjC.cpp
+++ b/lib/Sema/SemaDeclObjC.cpp
@@ -820,7 +820,7 @@
                       IdentifierInfo *CatName, SourceLocation CatLoc) {
   ObjCInterfaceDecl *IDecl = getObjCInterfaceDecl(ClassName, ClassLoc, true);
   ObjCCategoryDecl *CatIDecl = 0;
-  if (IDecl) {
+  if (IDecl && IDecl->hasDefinition()) {
     CatIDecl = IDecl->FindCategoryDeclaration(CatName);
     if (!CatIDecl) {
       // Category @implementation with no corresponding @interface.