rtsp: parse the correct token for the status code

Don't call get_token_len on next_token *and* pass in next_token to store the
subsequent pointer - the token we want to parse is the *current* value of
next_token, not the next next token (which may be beyond the end of the buffer,
if next_token happens to be the *last* token).

Bug: 10381
Change-Id: I9fb27e8bdaf2f9556f61841de30cec04b98ffb96
Reviewed-on: https://code.wireshark.org/review/3663
Reviewed-by: Evan Huus <eapache@gmail.com>