row_ext_create(): Allocate enough space for the structure.

This error was introduced in r867.

include/row0ext.ic

@@ -34,7 +34,8 @@ row_ext_create(
 	ulint		zip_size,/* compressed page size, or 0 */
 	mem_heap_t*	heap)	/* in: heap where created */
 {
-	row_ext_t*	ret = mem_heap_alloc(heap, (sizeof *ret) - 1 + n_ext);
+	row_ext_t*	ret = mem_heap_alloc(heap, (sizeof *ret)
+					     + (n_ext - 1) * sizeof ret->len);
 
 	ret->n_ext = n_ext;
 	ret->ext = ext;