Segfault when calling _lf_check_deadline

[housengw]

The root cause of the segfault is thatself->executing_reaction is NULL inside _lf_check_deadline, so when the check deadline function tries to dereference self->executing_reaction to get the deadline, it dereferences a NULL pointer.

Minimal Example to reproduce:

target C;

main reactor {
    reaction(startup) {=
        lf_check_deadline(self, true);
    =}
}

Running this code gives a segfault.
This bug has caused the AnytimePrime.lf example in the experimental repo to fail.