hotplug: network-bridge: fix for interfaces with no gateway

hotplug: network-bridge: fix for interfaces with no gateway

This comes from an old Debian bug report
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=588477 which refers to
http://lists.xen.org/archives/html/xen-users/2010-06/msg00420.html.

Although we no longer have a trap in _claim_lock as described (this was removed
in 25590:bb250383a4f5 "hotplug/Linux: use flock based locking") and the use of
network-* scripts is discouraged (in favour of using distro provided
mechanisms) the general principal that relying on the semantics of /bin/sh's &&
is unwise seems sound.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>