IP: try to fix OSX 10.5 x86 compilation

packet-ip.c: In function 'dissect_ip':
packet-ip.c:2297: warning: 'cur_rt' may be used uninitialized in this function
packet-ip.c:2297: note: 'cur_rt' was declared here

Change-Id: I97fe33d2a01e982f057cce6d68f64378727be2da
Reviewed-on: https://code.wireshark.org/review/6073
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>