[IPv6addr] Handle scanf failures in scan_if()
If for some reason the file being scanned is malformed, or
overflows one of the feilds for some reason, scanf will neither
return EOF nor fill in all of its parameters correctly. In the
case that I observed this resulted in an endless loop.

This simple fix just bails out in this case.
Perhaps an error message is in order?

--HG--
extra : convert_revision : 31d19acfd1b95134d7484f3ba72425f492a808d5