segfault in entry_guards_parse_state()
I got:
```
Program received signal SIGSEGV, Segmentation fault.
entry_guards_parse_state (state=state@entry=0x5555558e4840, set=set@entry=0, msg=0x7fffffffe1f0) at src/or/circuitbuild.c:4913
4913 node->first_hops = hop_cnt;
```
It seems that if there is an `EntryGuardPathBias` line without an `EntryGuard` line before it, it will try to do `node->first_hops = hop_cnt` with `node` being NULL, since `node` is instantiated when the first `EntryGuard` line is encountered.
Other codepaths in the same function, like `EntryGuardDownSince`, check for `!node` but it seems that `EntryGuardPathBias` doesn't. Checking for `!node` might be a sufficient fix.
issue