Error with watermark using ImageMagick with overlays larger than source image

My watermarking actions were failing if the source image was smaller than the watermark image. This caused errors like this in the logs:

ImageMagick reported error code 1. Message: convert: invalid argument for option `+-12+136': -geometry. in /var/www/maumusic.com.au/sites/default/modules/imageapi/imageapi_imagemagick.module on line 235.


Note the "+-" on the geometry offsets. The attached patch (tested on 6.x-1.9) resolves the issue by only prepending a + if the offset is equal or greater than 0 for x and y values.