Patch for histogram/stat2d.c

[Brian Gough]

Achim Gaedke writes:
 >  If wi and W is 0, linux prints 0 - but Solaris 5.7 prints NaN .
 > If wi is 0 and W is not, both print 0.

It seems to depend on the version of gcc too.  I think will move
towards remove the use of long double internally in the library -- it
is only used in a few places as an accumulator in loops to avoid
overflow in intermediate results.  With pure double precision the
results will be more reproducible in terms of IEEE arithmetic.

I'll make a prerelease for testing in future.  Previously I hadn't
encountered anything like that -- I compile the code on all the
different machines in the Sourceforge compile farm, but apparently
that doesn't catch everything.

Brian