Fix potential infinite loop when iterating over redeclarations of an...

Fix potential infinite loop when iterating over redeclarations of an ObjMethodDecl, resulting from invalid code.

Check for invalid decls in ObjCMethodDecl::getNextRedeclaration(); otherwise if we start from an invalid redeclaration
of an @implementation we would move to the @interface and not reach the original declaration again.

Fixes rdar://14024851

llvm-svn: 182951