Use constants not numbers

This patch uses warning/fatal constants instead of numbers with comments for
warning/alerts in d1_pkt.c and s3_pkt.c

RT#3725
Reviewed-by: NRich Salz <rsalz@openssl.org>

ssl/d1_pkt.c

@@ -1088,7 +1088,7 @@ int dtls1_read_bytes(SSL *s, int type, unsigned char *buf, int len, int peek)
             cb(s, SSL_CB_READ_ALERT, j);
         }
 
-        if (alert_level == 1) { /* warning */
+        if (alert_level == SSL3_AL_WARNING) {
             s->s3->warn_alert = alert_descr;
             if (alert_descr == SSL_AD_CLOSE_NOTIFY) {
 #ifndef OPENSSL_NO_SCTP
@@ -1137,7 +1137,7 @@ int dtls1_read_bytes(SSL *s, int type, unsigned char *buf, int len, int peek)
                 }
             }
 #endif
-        } else if (alert_level == 2) { /* fatal */
+        } else if (alert_level == SSL3_AL_FATAL) {
             char tmp[16];
 
             s->rwstate = SSL_NOTHING;

ssl/s3_pkt.c

@@ -1451,7 +1451,7 @@ int ssl3_read_bytes(SSL *s, int type, unsigned char *buf, int len, int peek)
             cb(s, SSL_CB_READ_ALERT, j);
         }
 
-        if (alert_level == 1) { /* warning */
+        if (alert_level == SSL3_AL_WARNING) {
             s->s3->warn_alert = alert_descr;
             if (alert_descr == SSL_AD_CLOSE_NOTIFY) {
                 s->shutdown |= SSL_RECEIVED_SHUTDOWN;
@@ -1474,7 +1474,7 @@ int ssl3_read_bytes(SSL *s, int type, unsigned char *buf, int len, int peek)
             else if (alert_descr == SSL_AD_MISSING_SRP_USERNAME)
                 return (0);
 #endif
-        } else if (alert_level == 2) { /* fatal */
+        } else if (alert_level == SSL3_AL_FATAL) {
             char tmp[16];
 
             s->rwstate = SSL_NOTHING;