[shape-plan] Fix typo!

The only effect is, if shaper_list was not NULL and no shaper was found,
we now don't insert anything into cache and return earlier.

src/hb-shape-plan.cc

diff --git a/src/hb-shape-plan.cc b/src/hb-shape-plan.cc
index e5ccc16..2166173 100644
--- a/src/hb-shape-plan.cc
+++ b/src/hb-shape-plan.cc
@@ -435,7 +435,7 @@
 #include "hb-shaper-list.hh"
 #undef HB_SHAPER_IMPLEMENT
 
-    if (unlikely (!proposal.shaper_list))
+    if (unlikely (!proposal.shaper_func))
       return hb_shape_plan_get_empty ();
   }