[Objective-c] Fix a crash that occurs when ObjCTypeParamList::back() is

called on an empty list.

This commit makes Parser::parseObjCTypeParamListOrProtocolRefs return
nullptr if it sees an invalid type parameter (e.g., __kindof) in the
type parameter list.

rdar://problem/23068920

Differential Revision: http://reviews.llvm.org/D15463

llvm-svn: 255754