Support dissecting REAL (BER) data values

Both exponent and 'integer N' values are limited:
 * max exponent is 3 octets/24-bits
 * max integer N is 8 octets/64-bit

Tested with zero value/length, integers, doubles, positive and negative numbers all using the Basic Encoding Rules (BER)

Change-Id: If92e1b3e209c42909b8cb76e6f50b8e6cd1da0da
Reviewed-on: https://code.wireshark.org/review/5527
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>