pcapng: don't leak block option strings

I *think* I got all the cases; I got most of them, at any rate, and enough to
shut up valgrind in all the test cases I ran.

Change-Id: I393bac0756f577b65e400b792f6719fa6ec4056a
Reviewed-on: https://code.wireshark.org/review/4244
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Evan Huus <eapache@gmail.com>