[Bug libstdc++/54351] ~unique_ptr() should not set __p to null

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54351

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-08-22 
18:47:07 UTC ---
That said, whether the testcase is valid or not, I don't see any harm in making
the change.