ST_AddBand is not null safe

[robe]

I was able to crash the postgres daemon when one of my procedures produced something like this.

SELECT ST_AddBand(ST_MakeEmptyRaster(271,271, 0, 0, 1, -1,0,0) , NULL::raster );

It should just throw an error rather than crashing the postgres daemon I think.

[dustymugs]

Fixed in r8303