Fix NaN double to long double conversion on VMS.

Promotion from double to long double on VMS is apparently done via
the math routine OTS$CVT_FLOAT_T_X, which seems to do the right
thing for ordinary values, infinities, and signaling NaNs.  But
for quiet NaNs (and only on Itanium, not Alpha) it produces a
negative infinity rather than a NaN.  Oops.

So hack around that by detecting NaN in the double and explicitly
setting it in the long double.  We can't use NV_NAN because this
is the code path in Perl_sv_vcatpvfn_flags where we have not
configured long doubles but are using the "fv" long double to format
output.