Calling malloc from signal handler

[nathanwbrei]

Signal handler is calling malloc. Signal handlers should avoid all non-async-signal-safe (approx the same thing as non-reentrant) syscalls as per https://man7.org/linux/man-pages/man7/signal-safety.7.html. That includes malloc and all IO.

If the segfault corrupts malloc itself, such as here (eic/EICrecon#402), we lose the backtrace.

The solution is to pre-allocate a buffer for the backtrace instead.