avoid false negatives on atoms and int resources used as string pointers

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on July 13, 2011 11:52:13_

to avoid false negatives with pointers that really are bogus and
unaddressable and not a legitimate atom or int resource we could query to
determine validity

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=488_