variable.c: improve binary search logic in bsearch_idx

Co-authored-by: Gemini <gemini@google.com>

src/variable.c

@@ -88,7 +88,7 @@ bsearch_idx(mrb_sym *keys, int size, mrb_sym target) {
     n -= half;
   }
   /* Final adjustment: if the remaining element is still less than target, advance by one */
-  return (int)(p - keys) + (keys[0] < target);
+  return (int)(p - keys) + (p[0] < target);
 }
 
 /* Set (insert or update) the value for `sym` in the instance variable table using branch-free search. */