sigcomp-udvm: fix invalid access

Way back in 2006, svn r20041 (now gd408f2f6fc) worked around a problem by
limiting the amount of buffer dealt with in the udvm_state_access call (changing
it from state_minimum_access_length_buff[n] to STATE_MIN_ACCESS_LEN).

The very next line however, tries to read the same amount of buffer to display
to the user - since it seems nobody ever applied a proper fix for the original
case, just apply the same workaround to the second call.

Bug: 10662
Change-Id: I32c81ee2a8eb7b30ff7ab1fbbb6ea5f206385d4b
Reviewed-on: https://code.wireshark.org/review/5124
Reviewed-by: Evan Huus <eapache@gmail.com>