DECT: ensure that bfield_data buffer is always fully initiated

Bug: 10710
Change-Id: I444bc4ff7e542a8aa5ac8e20bac44a823ac74cfd
Reviewed-on: https://code.wireshark.org/review/5371
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>