__atomic_futex_unsigned::_M_load_when_not_equal has a 'return' statement with no value, in function returning unsigned int

[Patrick Palka]

Specifically, line 149 of atomix_futex.h has a bare "return;"
statement, but the function is marked as returning non-void.  This was
caught while working on PR c++/18969.  Vanilla G++ does not catch this
error because return statements inside templates are currently only
analyzed during instantiation time.