bgpd: Make sure we pop the table from the stack when decoding struct attr

bgpd: Make sure we pop the table from the stack when decoding struct attr

```
bgpd[81718]: bgpd/bgp_routemap.c:382: route_match_script(): assertion (lua_gettop(lfs->L) == 1) failed
BGP[81718]: Received signal 6 at 1648554165 (si_addr 0x6e00013f36, PC 0x7fcc5adebce1); aborting...
BGP[81718]: /usr/local/lib/libfrr.so.0(zlog_backtrace_sigsafe+0x5e) [0x7fcc5b1caf5e]
BGP[81718]: /usr/local/lib/libfrr.so.0(zlog_signal+0xe6) [0x7fcc5b1cb136]
BGP[81718]: /usr/local/lib/libfrr.so.0(+0xcd4b2) [0x7fcc5b1f54b2]
BGP[81718]: /lib/x86_64-linux-gnu/libpthread.so.0(+0x14140) [0x7fcc5af89140]
BGP[81718]: /lib/x86_64-linux-gnu/libc.so.6(gsignal+0x141) [0x7fcc5adebce1]
BGP[81718]: /lib/x86_64-linux-gnu/libc.so.6(abort+0x123) [0x7fcc5add5537]
BGP[81718]: /usr/local/lib/libfrr.so.0(_zlog_assert_failed+0xd7) [0x7fcc5b21ecd7]
BGP[81718]: /usr/lib/frr/bgpd(+0x162710) [0x55ed70255710]
BGP[81718]: /usr/local/lib/libfrr.so.0(+0xc0e47) [0x7fcc5b1e8e47]
BGP[81718]: /usr/local/lib/libfrr.so.0(route_map_apply_ext+0x4b6) [0x7fcc5b1eaaf6]
BGP[81718]: /usr/lib/frr/bgpd(+0x13eb24) [0x55ed70231b24]
BGP[81718]: /usr/lib/frr/bgpd(bgp_update+0x7db) [0x55ed7023f81b]
BGP[81718]: /usr/lib/frr/bgpd(bgp_nlri_parse_ip+0x113) [0x55ed70241bc3]
```

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