ovsdb: Fix double-free in ovsdb_jsonrpc_session_close().

ovsdb: Fix double-free in ovsdb_jsonrpc_session_close().

ovsdb_session_destroy() was called twice but it should only be called once.

This double-free is unlikely to cause problems in practice because it only
triggers if there were ever more than two outstanding requests in the
session at a time (because the only data being freed is an hmap, which
does not allocate any heap memory unless the hmap has more than two
elements).

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>