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.