BUG/MINOR: ssl: fix smp_fetch_ssl_fc_session_id

Dmitry Sivachenko reported the following build warning using Clang
which is a real bug :

src/ssl_sock.c:4104:44: warning: address of 'smp->data.str.len' will always
      evaluate to 'true' [-Wpointer-bool-conversion]
        if (!smp->data.str.str || !&smp->data.str.len)

The impact is very low however, it will return an empty session_id
instead of no session id when none is found.

The fix should be backported to 1.5.
(cherry picked from commit 745d4127582a8c66e2e8ce35f746a78e867960af)
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 93aab8b..7d77d36 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -3540,7 +3540,7 @@
 		return 0;
 
 	smp->data.str.str = (char *)SSL_SESSION_get_id(sess, (unsigned int *)&smp->data.str.len);
-	if (!smp->data.str.str || !&smp->data.str.len)
+	if (!smp->data.str.str || !smp->data.str.len)
 		return 0;
 
 	return 1;