Add some additional safety checks to the StructuredData access

methods - lldb can still crash pretty easily on corrupt JSON text,
and these will help eliminate a bunch of cases where that would
result in a crash.  Some of the methods would check that e.g.
GetItemAtIndex would actually return an item before dereferencing it,
some would not, that kind of thing.  

<rdar://problem/23768693> 

llvm-svn: 255093