rpc: fix regression in gbfc5483174d

value can be NULL, in which case memduping it will crash

Bug: 10667
Change-Id: I8950fae94d80a2f06f24a785532536ad73f1855d
Reviewed-on: https://code.wireshark.org/review/5152
Reviewed-by: Evan Huus <eapache@gmail.com>