Don't #reprocess! if there is no file.

lib/paperclip/attachment.rb

@@ -159,7 +159,7 @@ module Paperclip
     # again.
     def reprocess!
       new_original = Tempfile.new("paperclip-reprocess")
-      old_original = to_file(:original)
+      if old_original = to_file(:original)
         new_original.write( old_original.read )
         new_original.rewind
 
@@ -168,6 +168,7 @@ module Paperclip
 
         old_original.close if old_original.respond_to?(:close)
       end
+    end
 
     private