BUG/MEDIUM: ssl: Don't attempt to set alpn if we're not using SSL.

Checks use ssl_sock_set_alpn() to set the ALPN if check-alpn is used, however
check-alpn failed to check if the connection was indeed using SSL, and thus,
would crash if check-alpn was used on a non-SSL connection. Fix this by
making sure the connection uses SSL before attempting to set the ALPN.

This should be backported to 2.0 and 1.9.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 0524006..c9fffbe 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -6411,6 +6411,9 @@
 #ifdef TLSEXT_TYPE_application_layer_protocol_negotiation
 	struct ssl_sock_ctx *ctx = conn->xprt_ctx;
 
+	if (!ssl_sock_is_ssl(conn))
+		return;
+
 	SSL_set_alpn_protos(ctx->ssl, alpn, len);
 #endif
 }