Fix initialization of s->once, which gcc accepts in C++ mode, but clang does not.  Setting to zero is ok, because nsync_once always allows zero to be a valid initial value.
Fix initialization of s->once, which gcc accepts in C++ mode, but clang does
not.  Setting to zero is ok, because nsync_once always allows zero to be a
valid initial value.

google/nsync,google/nsync,google/nsync,google/nsync