append the interface index instead of throwing an exception when the scope id lookup...

append the interface index instead of throwing an exception when the scope id lookup fails

Summary: As written, this str() method depends on the current state of the host - if an interface disappears (as is commong with eg. mobile, ipsec, etc.) then attempting to generate a string representation of this v6 address will currently throw an exception. Instead, we're going toignore the lookup failure and append the index i; this form is interchangeable, though slightly less legible.

Reviewed By: yfeldblum

Differential Revision: D5443618

fbshipit-source-id: bbd3b4edaee8098671140af7400377d5e2b1937b