Cleanly fetch the query string.

Cleanly fetch the query string.

Use tvb_get_string_enc(), because it guarantees null termination, unlike
tvb_memdup(), and use ENC_ASCII|ENC_NA as the encoding, to convert
non-ASCII characters to Unicode REPLACEMENT CHARACTER (there is *no*
standard for the encoding of WHOIS texts other than "ASCII"; we use
ENC_ASCII|ENC_NA elsewhere).

Bug: 16298
Change-Id: I6b7ff2a1af3997e8426ca07bea1ce68fe9cbc9c6
Reviewed-on: https://code.wireshark.org/review/35564
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <guy@alum.mit.edu>