better mask sizing for gaussmat

We were calculating the mask size incorrectly for small masks.

Thanks johntrunc

see https://github.com/libvips/libvips/issues/1793

ChangeLog

@@ -5,6 +5,7 @@
 - stop 0-length buffer being passed to imagemagick [lovell]
 - convert no-profile CMYK to RGB on save [augustocdias]
 - ensure SVG loader skips input with chars outside x09-x7F range [lovell]
+- better mask sizing in gaussmat [johntrunc]
 
 24/1/20 started 8.10.0
 - more conformat IIIF output from dzsave [regisrob]

libvips/create/gaussmat.c

@@ -129,7 +129,7 @@ vips_gaussmat_build( VipsObject *object )
 		vips_error( class->nickname, "%s", _( "mask too large" ) );
 		return( -1 );
 	}
-	width = 2 * x - 1;
+	width = 2 * VIPS_MAX( x - 1, 0 ) + 1;
 	height = gaussmat->separable ? 1 : width; 
 
 	vips_image_init_fields( create->out,