slirp: fork_exec(): Don't close() a negative number in fork_exec()

In a fork_exec() error path we try to closesocket(s) when s might
be a negative number because the thing that failed was the
qemu_socket() call. Add a guard so we don't do this.

(Spotted by Coverity: CID 1005727 issue 1 of 2.)
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>

slirp/misc.c

@@ -112,7 +112,9 @@ fork_exec(struct socket *so, const char *ex, int do_pty)
 		    bind(s, (struct sockaddr *)&addr, addrlen) < 0 ||
 		    listen(s, 1) < 0) {
 			error_report("Error: inet socket: %s", strerror(errno));
-			closesocket(s);
+			if (s >= 0) {
+			    closesocket(s);
+			}
 
 			return 0;
 		}