Don't confuse TLS bidirectional shutdown on record type zero.

The bidi shutdown code uses type = 0 as a special signal value, but code
elsewhere doesn't account for this.

BUG=526437

Change-Id: I090cee421633d70ef3b84f4daa811608031b9ed9
Reviewed-on: https://boringssl-review.googlesource.com/5771
Reviewed-by: Adam Langley <agl@google.com>

ssl/s3_pkt.c

@@ -458,7 +458,7 @@ start:
     return 0;
   }
 
-  if (type == rr->type) {
+  if (type != 0 && type == rr->type) {
     s->s3->warning_alert_count = 0;
 
     /* SSL3_RT_APPLICATION_DATA or SSL3_RT_HANDSHAKE */