fix linear with mismatched vectors

we had:

	$ vips linear sarto_ng.v x.v "1 2 3" "4"
	$ vipsheader x.v
	x.v: 1865x2556 float, 1 band, b-w
	$ vips linear sarto_ng.v x.v "1" "4 5 6"
	$ vipsheader x.v
	x.v: 1865x2556 float, 3 bands, b-w

there was a typo in linear.c