Write heartbeat thread output to the safe crash log

In `jl_safe_printf()`, we're already writing to the safe crash log
if we're in signal handler context (in addition to writing to
`stderr`). Now we do the same if we're in the heartbeat thread.

src/jl_uv.c

@@ -747,7 +747,7 @@ JL_DLLEXPORT void jl_safe_printf(const char *fmt, ...)
     // order is important here: we want to ensure that the threading infra
     // has been initialized before we start trying to print to the
     // safe crash log file
-    if (jl_sig_fd != 0 && jl_inside_signal_handler()) {
+    if (jl_sig_fd != 0 && (jl_inside_signal_handler() || jl_inside_heartbeat_thread()) {
         print_error_msg_as_json(buf);
     }
     if (write(STDERR_FILENO, buf, strlen(buf)) < 0) {