packet-vnc.c: Restore tvb_get_string() --> tvb_get_string_enc() changes lost in gdf6bdcc

  These changes were originally done in g971ffd6

Change-Id: I9de28ba7089f99e8058207f3b6d34de931decf76
Reviewed-on: https://code.wireshark.org/review/2835
Reviewed-by: Bill Meier <wmeier@newsguy.com>