mysqld_exit(): Change the assertion failure on
global_status_var.global_memory_used == 0
to fprintf, like in 0bcb65d3

It appears that in some cases, that variable may be nonzero
even when LeakSanitizer (WITH_ASAN) would not report errors.
This was observed in 10.4 88cf6f1c
with the MDEV-22348 test case (Aria startup failure when running
main.default_storage_engine).