Have buffer_free() null out the buffer data pointer.

That prevents some double-free issues (I got one when doing non-"Update
list of packets in real time" captures, if I do one such capture and
then another one).

Change-Id: Ia08034d9d1640bad21b74960efade8926dbfc5de
Reviewed-on: https://code.wireshark.org/review/3063
Reviewed-by: Guy Harris <guy@alum.mit.edu>