Fix a warning for a debug assertion

The type uint16_t is apparently promoted to int in a comparison,
causing a sign mismatch when comparing to ulint.
Convert both operands to uint16_t.

storage/innobase/include/page0page.ic

@@ -731,7 +731,7 @@ page_dir_set_n_heap(
 	ulint		n_heap)	/*!< in: number of records */
 {
 	ut_ad(n_heap < 0x8000);
-	ut_ad(!page_zip || n_heap
+	ut_ad(!page_zip || uint16_t(n_heap)
 	      == (page_header_get_field(page, PAGE_N_HEAP) & 0x7fff) + 1);
 
 	page_header_set_field(page, page_zip, PAGE_N_HEAP, n_heap