staticd: Do not crash when modifying an existing static route with color

staticd: Do not crash when modifying an existing static route with color

```
$ vtysh -c 'conf' -c 'ip route 192.168.0.0/32 192.168.10.65 color 123'
$ vtysh -c 'show ip route 192.168.0.0/32 json' | jq '."192.168.0.0/32"[0].nexthops[0].srteColor'
123
$ vtysh -c 'conf' -c 'ip route 192.168.0.0/32 192.168.10.65'
$ vtysh -c 'show ip route 192.168.0.0/32 json' | jq '."192.168.0.0/32"[0].nexthops[0].srteColor'
0
```

Before this patch:
```
STATIC: Received signal 6 at 1666938633 (si_addr 0x4e1a, PC 0x7f295139c438); aborting...
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(zlog_backtrace_sigsafe+0x70) [0x7f29519ef0a0]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(zlog_signal+0xec) [0x7f29519ef28c]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xccee2) [0x7f2951a1aee2]
STATIC: /lib/x86_64-linux-gnu/libpthread.so.0(+0x11390) [0x7f2951742390]
STATIC: /lib/x86_64-linux-gnu/libc.so.6(gsignal+0x38) [0x7f295139c438]
STATIC: /lib/x86_64-linux-gnu/libc.so.6(abort+0x16a) [0x7f295139e03a]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(_zlog_assert_failed+0xe6) [0x7f2951a46766]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xae7ee) [0x7f29519fc7ee]
STATIC: /home/u1/mGIT/frr-master/staticd/.libs/lt-staticd(routing_control_plane_protocols_control_plane_protocol_staticd_route_list_src_
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xaf264) [0x7f29519fd264]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xafb4f) [0x7f29519fdb4f]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(nb_candidate_commit_apply+0x33) [0x7f29519fde33]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(nb_candidate_commit+0x9e) [0x7f29519fdf6e]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xb0469) [0x7f29519fe469]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xb3055) [0x7f2951a01055]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(nb_cli_apply_changes+0xd8) [0x7f2951a01358]
STATIC: /home/u1/mGIT/frr-master/staticd/.libs/lt-staticd() [0x407ec2]
STATIC: /home/u1/mGIT/frr-master/staticd/.libs/lt-staticd() [0x40a307]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0x74923) [0x7f29519c2923]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(cmd_execute_command+0xe4) [0x7f29519c2aa4]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(cmd_execute+0xc9) [0x7f29519c2c49]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xe4184) [0x7f2951a32184]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xe4436) [0x7f2951a32436]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(+0xe6fcc) [0x7f2951a34fcc]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(thread_call+0x98) [0x7f2951a2cc28]
STATIC: /home/u1/mGIT/frr-master/lib/.libs/libfrr.so.0(frr_run+0xe8) [0x7f29519e7518]
STATIC: /home/u1/mGIT/frr-master/staticd/.libs/lt-staticd(main+0xda) [0x40572a]
STATIC: /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7f2951387840]
STATIC: /home/u1/mGIT/frr-master/staticd/.libs/lt-staticd(_start+0x29) [0x4058e9]
STATIC: in thread vtysh_read scheduled from lib/vty.c:2710 vty_event()
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>