fix a warning with gcc 4.7.2 on linux

packet-isakmp.c: In function 'dissect_isakmp':
packet-isakmp.c:1873:6: error: 'ivd' may be used uninitialized in this function [-Werror=maybe-uninitialized]
packet-isakmp.c:1747:14: note: 'ivd' was declared here
cc1: all warnings being treated as errors

(not sure why the compiler complains here)

Change-Id: I9b9589d3193b0dc37f2db8f6c4c6727b2a92eaaa
Reviewed-on: https://code.wireshark.org/review/3444
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>