fix: allow changing clip_skip back to its default value

CLIPTextModel ignored setting clip_skip back to -1,
instead retaining whatever value was previously defined.

This is not relevant to command-line sd, since it's currently
not possible to change clip_skip between generations, but
could affect any frontend that reuses model instances for
multiple images.

Author: wbruna

clip.hpp

@@ -701,7 +701,7 @@ class CLIPTextModel : public GGMLBlock {
 
     void set_clip_skip(int skip) {
         if (skip <= 0) {
-            return;
+            skip = -1;
         }
         clip_skip = skip;
     }