Avoids panic when checking for conflicts against an uninitalized network

A network is added to the `d.networks` map before it's fully initialized. That
is, it's possible for a network in `d.networks` to exist without having
`bridgeIPv4` populated yet. If multiple networks are spun up close to the same
time, a panic can occur.

Example:
```
panic(0x1a75d20, 0xc82000e090)
        /usr/local/go/src/runtime/panic.go:443 +0x4e9
net.networkNumberAndMask(0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
        /usr/local/go/src/net/ip.go:433 +0x42
net.(*IPNet).Contains(0x0, 0xc82084dbd0, 0x4, 0x4, 0xc820010200)
        /usr/local/go/src/net/ip.go:457 +0x25
github.com/docker/libnetwork/drivers/bridge.(*networkConfiguration).conflictsWithNetworks(0xc822249360, 0xc822761380, 0x40, 0xc820866a60, 0x4, 0x4, 0x0, 0x0)
        /root/rpmbuild/BUILD/docker-engine/vendor/src/github.com/docker/libnetwork/drivers/bridge/bridge.go:334 +0x40b
```

Signed-off-by: Andy Lindeman <alindeman@salesforce.com>

libnetwork/drivers/bridge/bridge.go

@@ -330,7 +330,7 @@ func (c *networkConfiguration) conflictsWithNetworks(id string, others []*bridge
 		// bridges. This could not be completely caught by the config conflict
 		// check, because networks which config does not specify the AddressIPv4
 		// get their address and subnet selected by the driver (see electBridgeIPv4())
-		if c.AddressIPv4 != nil {
+		if c.AddressIPv4 != nil && nwBridge.bridgeIPv4 != nil {
 			if nwBridge.bridgeIPv4.Contains(c.AddressIPv4.IP) ||
 				c.AddressIPv4.Contains(nwBridge.bridgeIPv4.IP) {
 				return types.ForbiddenErrorf("conflicts with network %s (%s) by ip network", nwID, nwConfig.BridgeName)