server: fix nil pointer exception in evpn mac mobility

With large EVPN fabrics, it can happen that some paths are nil in the
mac mobility codepath, causing crashes of the process.

    panic: runtime error: invalid memory address or nil pointer dereference
    [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xb8e03a]
    goroutine 25 [running]:
    github.com/osrg/gobgp/v3/internal/pkg/table.(*Path).root(...)
        /home/tuetuopay/dev/gobgp/internal/pkg/table/path.go:341
    github.com/osrg/gobgp/v3/internal/pkg/table.(*Path).OriginInfo(...)
        /home/tuetuopay/dev/gobgp/internal/pkg/table/path.go:348
    github.com/osrg/gobgp/v3/internal/pkg/table.(*Path).GetNlri(...)
        /home/tuetuopay/dev/gobgp/internal/pkg/table/path.go:460
    github.com/osrg/gobgp/v3/pkg/server.getMacMobilityExtendedCommunity(0x0, {0xc002833520, 0x6, 0xc00033ec00?}, {0xc001a52008, 0xa72, 0xc000000000?})
        /home/tuetuopay/dev/gobgp/pkg/server/server.go:2044 +0x1da
    github.com/osrg/gobgp/v3/pkg/server.(*BgpServer).fixupApiPath(0xc000372008, {0x0, 0x0}, {0xc0008fbe48, 0x1, 0x0?})
        /home/tuetuopay/dev/gobgp/pkg/server/server.go:2120 +0x4bc
    github.com/osrg/gobgp/v3/pkg/server.(*BgpServer).addPathList(0xc000372008, {0x0?, 0x0?}, {0xc0008fbe48, 0x1, 0x1})
        /home/tuetuopay/dev/gobgp/pkg/server/server.go:2162 +0x2c
    github.com/osrg/gobgp/v3/pkg/server.(*BgpServer).AddPath.func1()
        /home/tuetuopay/dev/gobgp/pkg/server/server.go:2191 +0xa5
    github.com/osrg/gobgp/v3/pkg/server.(*BgpServer).handleMGMTOp(0xc0b4e0?, 0xc001243140)
        /home/tuetuopay/dev/gobgp/pkg/server/server.go:280 +0x82
    github.com/osrg/gobgp/v3/pkg/server.(*BgpServer).Serve(0xc000372008)
        /home/tuetuopay/dev/gobgp/pkg/server/server.go:490 +0x53a
    created by main.main in goroutine 1
        /home/tuetuopay/dev/gobgp/cmd/gobgpd/main.go:202 +0x13ea

pkg/server/server.go

@@ -2085,6 +2085,9 @@ func getMacMobilityExtendedCommunity(etag uint32, mac net.HardwareAddr, evpnPath
 	}, 0)
 
 	for _, path := range evpnPaths {
+		if path == nil {
+			continue
+		}
 		nlri := path.GetNlri().(*bgp.EVPNNLRI)
 		target, ok := nlri.RouteTypeData.(*bgp.EVPNMacIPAdvertisementRoute)
 		if !ok {