AST-2017-010: Fix cdr_object_update_party_b_userfield_cb() buf overrun

cdr_object_update_party_b_userfield_cb() could overrun the fixed buffer if
the supplied string is too long.  The long string could be supplied by
external means using the CDR(userfield) function.

This may seem reminiscent to AST-2017-001 (ASTERISK_26897) and it is.  The
earlier patch fixed the buffer overrun for Party A's userfield while this
patch fixes the same thing for Party B's userfield.

ASTERISK-27337

Change-Id: I0fa767f65ecec7e676ca465306ff9e0edbf3b652