hw/9pfs: Fix potential memory leak and avoid reuse of freed memory

The leak was reported by cppcheck.

Function proxy_init also calls g_free for ctx->fs_root.
Avoid reuse of this memory by setting ctx->fs_root to NULL.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: M. Mohan Kumar <mohan@in.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
