bgpd: Check the actual remaining stream length before taking TLV value

bgpd: Check the actual remaining stream length before taking TLV value

```
    0 0xb50b9f898028 in __sanitizer_print_stack_trace (/home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/.libs/bgpd+0x368028) (BuildId: 3292703ed7958b20076550c967f879db8dc27ca7)
    1 0xb50b9f7ed8e4 in fuzzer::PrintStackTrace() (/home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/.libs/bgpd+0x2bd8e4) (BuildId: 3292703ed7958b20076550c967f879db8dc27ca7)
    2 0xb50b9f7d4d9c in fuzzer::Fuzzer::CrashCallback() (/home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/.libs/bgpd+0x2a4d9c) (BuildId: 3292703ed7958b20076550c967f879db8dc27ca7)
    3 0xe0d12d7469cc  (linux-vdso.so.1+0x9cc) (BuildId: 1a77697e9d723fe22246cfd7641b140c427b7e11)
    4 0xe0d12c88f1fc in __pthread_kill_implementation nptl/pthread_kill.c:43:17
    5 0xe0d12c84a678 in gsignal signal/../sysdeps/posix/raise.c:26:13
    6 0xe0d12c83712c in abort stdlib/abort.c:79:7
    7 0xe0d12d214724 in _zlog_assert_failed /home/ubuntu/frr-public/frr_public_private-libfuzzer/lib/zlog.c:789:2
    8 0xe0d12d1285e4 in stream_get /home/ubuntu/frr-public/frr_public_private-libfuzzer/lib/stream.c:324:3
    9 0xb50b9f8e47c4 in bgp_attr_encap /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_attr.c:2758:3
    10 0xb50b9f8dcd38 in bgp_attr_parse /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_attr.c:3783:10
    11 0xb50b9faf74b4 in bgp_update_receive /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:2383:20
    12 0xb50b9faf1dcc in bgp_process_packet /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:4075:11
    13 0xb50b9f8c90d0 in LLVMFuzzerTestOneInput /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_main.c:582:3
```

Reported-by: Iggy Frankovic <iggyfran@amazon.com>
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>