slirp: Properly initialize pollfds_idx of new sockets

Otherwise we may start processing sockets in slirp_pollfds_poll that
were created past slirp_pollfds_fill.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

slirp/socket.c

diff --git a/slirp/socket.c b/slirp/socket.c
index a7ab933..bb639ae 100644
--- a/slirp/socket.c
+++ b/slirp/socket.c
@@ -51,6 +51,7 @@
     so->so_state = SS_NOFDREF;
     so->s = -1;
     so->slirp = slirp;
+    so->pollfds_idx = -1;
   }
   return(so);
 }