Prevent ST_MapAlgebra segfault specifying rast2 with a single raster

[Regina Obe]

> I had a copy-paste error go a little bit more wrong than I'd have expected.
> Patch attached with test, but I don't know if this is the best way/place to fix
> the issue.

Dian,

Thanks for the patch  and ticket -  https://trac.osgeo.org/postgis/ticket/5785

At a glance guarding for case greater than 7 (raster 2 args) in that spot seems good enough to me.

I've back ported thru 3.3.8.

Could probably go to 3.2.  I'll revisit that later.

Thanks,
Regina