Fix T100142: Compare correct render_slot variable during clear operation

In {rB0ef8a6179d2a773b2570352bd0cb7eb18b666da2} the parameter name was
changed to match the header declaration (slot) but it missed updating
the variable name inside the function correctly in one instance.

This prevents slot 0 from being cleared if the last slot to be rendered
was not also 0.

Differential Revision: https://developer.blender.org/D15620

source/blender/blenkernel/intern/image.cc

@@ -5516,7 +5516,7 @@ bool BKE_image_clear_renderslot(Image *ima, ImageUser *iuser, int slot)
   }
 
   RenderSlot *render_slot = static_cast<RenderSlot *>(BLI_findlink(&ima->renderslots, slot));
-  if (!slot) {
+  if (!render_slot) {
     return false;
   }
   if (render_slot->render) {