array-map! no source args

[Kevin Ryde]

array-map! got changed to demand at least one source arg, in response
to a bug report about it segfaulting on a certain case of no args.

I think I'll try to change it back to accept no sources, because that
worked in 1.4, it's still in scm, and slib is using that feature.  I
suspect it might be as simple as after it dispatches on the subr type
of the proc it should then check how many actual args there are.