Always ensure that init_msg is initialised for a CCS

We read it later in grow_init_buf(). If CCS is the first thing received in
a flight, then it will use the init_msg from the last flight we received. If
the init_buf has been grown in the meantime then it will point to some
arbitrary other memory location. This is likely to result in grow_init_buf()
attempting to grow to some excessively large amount which is likely to
fail. In practice this should never happen because the only time we receive
a CCS as the first thing in a flight is in an abbreviated handshake. None
of the preceding messages from the server flight would be large enough to
trigger this.

Reviewed-by: Rich Salz <rsalz@openssl.org>

ssl/statem/statem_lib.c

@@ -391,6 +391,7 @@ int tls_get_message_header(SSL *s, int *mt)
                 }
                 s->s3->tmp.message_type = *mt = SSL3_MT_CHANGE_CIPHER_SPEC;
                 s->init_num = readbytes - 1;
+                s->init_msg = s->init_buf->data;
                 s->s3->tmp.message_size = readbytes;
                 return 1;
             } else if (recvd_type != SSL3_RT_HANDSHAKE) {