A leak of the contents of fil_system.ssd that was introduced in
commit 10dd290b (MDEV-17380)
was caught by implementing SAFEMALLOC instrumentation of operator new.
I did not try to find out how to make AddressSanitizer or Valgrind
detect it.

fil_system_t::close(): Clear fil_system.ssd.

The leak was identified and a fix suggested by Michael Widenius
and Vicențiu Ciorbaru.