Several places in virInterfaceDefParseProtoIPv6 clobber the
default 'ret' return value. So when jumping to cleanup on
error, 'ret' may mistakenly be set to 0 instead of -1. This
caused failure to report OOM errors, meaning data was silently
lost during parsing.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>