[MGCP] When changing tvb_length(tvb) -> tvb_reported_length(tvb) it turns
out that tvb_new_subset(tvb, tvb_sectionbegin, sectionlen, -1) causes a
problem as reported_length is set to -1, set it to sectionlen.

Change-Id: I650d3fa1390be5add66ed44d0f70929b9eb0aad6
Reviewed-on: https://code.wireshark.org/review/5505
Reviewed-by: Anders Broman <a.broman58@gmail.com>