Avoid out-of-bounds write in SSL_get_shared_ciphers

PR: 3317

(Imported from upstream's 8571902e23 and
1f5bce2dce)

ssl/ssl_lib.c

@@ -1518,6 +1518,10 @@ char *SSL_get_shared_ciphers(const SSL *s,char *buf,int len)
 
 	p=buf;
 	sk=s->session->ciphers;
+
+	if (sk_SSL_CIPHER_num(sk) == 0)
+		return NULL;
+
 	for (i=0; i<sk_SSL_CIPHER_num(sk); i++)
 		{
 		int n;