Zero s->packet when releasing the read buffer.

s->packet points into the read buffer. It shouldn't leave a dangling pointer.

Change-Id: Ia7def2f50928ea9fca8cb0b69d614a92f9f47f57
Reviewed-on: https://boringssl-review.googlesource.com/4684
Reviewed-by: Adam Langley <agl@google.com>

ssl/s3_both.c

@@ -672,6 +672,7 @@ int ssl3_release_write_buffer(SSL *s) {
 int ssl3_release_read_buffer(SSL *s) {
   OPENSSL_free(s->s3->rbuf.buf);
   s->s3->rbuf.buf = NULL;
+  s->packet = NULL;
   return 1;
 }