[Bug libstdc++/59656] weak_ptr::lock function crashes when compiling with -fno-exceptions flag

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59656

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 31920
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31920&action=edit
don't use _M_add_ref_lock() when exceptions are disabled

Here's a patch which fixes Paul's testcase