Fix read_ahead issue

Fix a "&" that should have been "!" when processing read_ahead.

RT#3793
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

ssl/record/rec_layer_s3.c

@@ -380,7 +380,7 @@ int ssl3_read_n(SSL *s, int n, int max, int extend)
     }
 
     /* We always act like read_ahead is set for DTLS */
-    if (&s->rlayer.read_ahead && !SSL_IS_DTLS(s))
+    if (!s->rlayer.read_ahead && !SSL_IS_DTLS(s))
         /* ignore max parameter */
         max = n;
     else {