ex: Try to fix CID 500156 and CID 500157 - dereference after null check in atom.c

Try to fix CID 500156 and CID 500157 : Dereference after null check, in ntoskrnl/ex/atom.c
The idea is to know whether having a CapturedName == NULL is valid. After looking at RtlAddAtomToAtomTable and RtlLookupAtomInAtomTable functions (in lib/rtl/atom.c) one notes that they call subfunctions which dereference the AtomName (== CapturedName) parameter. The proposition of returning a STATUS_OBJECT_NAME_INVALID error is taken from the fact that, in RtlAddAtomToAtomTable, one returns it long after when "the caller supplied an empty atom name".