guard references

Author: jcupitt

lib/vips/image.rb

@@ -458,14 +458,14 @@ def self.new_from_source source, option_string, **opts
       loader = Vips.vips_foreign_find_load_source source
       raise Vips::Error if loader.nil?
 
-      image = Vips::Operation.call loader, [source], opts, option_string
+      result = Vips::Operation.call loader, [source], opts, option_string
 
       # keep a secret ref to the source object ... the libvips loader will
       # keep a ref to the C source object, but we need the ruby wrapper object
       # to stay alive too
-      image.references << source
+      result.references << source if result.is_a?(Vips::Image)
 
-      image
+      result
     end
 
     def self.matrix_from_array width, height, array