[Analyzer] Fix crash in ObjCPropertyChecker on protocol property

[dcoughlin]

Summary

Fix a crash in the ObjCPropertyChecker when analyzing a 'copy' property of an NSMutable* type in a protocol.

[jordan_rose]

Looks good to me. Good catch.

lib/StaticAnalyzer/Checkers/ObjCPropertyChecker.cpp

const std::string &PropTypeName(T->getPointeeType().getCanonicalType()
                                                      .getAsString());
   if (!StringRef(PropTypeName).startswith("NSMutable"))
     return;
 
   const ObjCImplDecl *ImplD = nullptr;
   if (const ObjCInterfaceDecl *IntD =
           dyn_cast<ObjCInterfaceDecl>(D->getDeclContext())) {
     ImplD = IntD->getImplementation();
-  } else {
-    const ObjCCategoryDecl *CatD = cast<ObjCCategoryDecl>(D->getDeclContext());
+  } else if (auto *CatD = dyn_cast<ObjCCategoryDecl>(D->getDeclContext())) {
     ImplD = CatD->getClassInterface()->getImplementation();
   }
 
   if (!ImplD || ImplD->HasUserDeclaredSetterMethod(D))
     return;
 
   SmallString<128> Str;
   llvm::raw_svector_ostream OS(Str);

test/Analysis/ObjCPropertiesSyntaxChecks.m

 @implementation J
 @end
 
 // If we do not see the implementation then we do not want to warn,
 // because we may miss a user-defined setter that works correctly.
 @interface IWithoutImpl : NSObject {}
 @property(copy) NSMutableString *mutableStr; // no-warning
 @end
+
+@protocol SomeProtocol
+// Don't warn on protocol properties because it is possible to
+// conform to them correctly; it is only synthesized setters that
+// that are definitely incorrect.
+@property (copy) NSMutableString *myProp; // no-crash // no-warning
+@end