coio: fix cord leak on stop

cord_ptr variable is calloc()-ated in coio_on_start()
and is not free()-ed, which triggers ASAN. free() it
in coio_on_stop().

Closes #5308

(cherry picked from commit 8477b6c)

asan/lsan.supp

@@ -43,8 +43,6 @@ leak:tt_bitset_iterator_init
 leak:libc.so*
 
 # test: box-tap/schema_mt.test.lua
-# source: src/lib/core/coio_task.c
-leak:coio_on_start
 # source: src/lib/salad/mhash.h
 leak:mh_i32ptr_new
 

src/lib/core/coio_task.c

@@ -123,6 +123,7 @@ coio_on_stop(void *data)
 {
 	(void) data;
 	cord_destroy(cord());
+	free(cord());
 	return 0;
 }