weston: arm SEGV handler earlier

It is useful to print the backtrace regardless of whether we have a
compositor and a backend initialized yet. Move catch_signals() to the
earliest point in main() and protect the SEGV handler from dereferencing
NULL when we don't yet have a compositor or a backend.

The SEGV handler uses weston_log(), so cannot move catch_signals() any
earlier.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>

compositor/main.c

@@ -656,6 +656,7 @@ on_caught_signal(int s, siginfo_t *siginfo, void *context)
 
 	print_backtrace();
 
+	if (segv_compositor && segv_compositor->backend)
 		segv_compositor->backend->restore(segv_compositor);
 
 	raise(SIGTRAP);
@@ -1824,6 +1825,8 @@ int main(int argc, char *argv[])
 	weston_log_set_handler(vlog, vlog_continue);
 	weston_log_file_open(log);
 
+	catch_signals();
+
 	weston_log("%s\n"
 		   STAMP_SPACE "%s\n"
 		   STAMP_SPACE "Bug reports to: %s\n"
@@ -1872,6 +1875,7 @@ int main(int argc, char *argv[])
 		weston_log("fatal: failed to create compositor\n");
 		goto out;
 	}
+	segv_compositor = ec;
 
 	if (weston_compositor_init_config(ec, config) < 0)
 		goto out;
@@ -1887,9 +1891,6 @@ int main(int argc, char *argv[])
 
 	weston_pending_output_coldplug(ec);
 
-	catch_signals();
-	segv_compositor = ec;
-
 	if (idle_time < 0)
 		weston_config_section_get_int(section, "idle-time", &idle_time, -1);
 	if (idle_time < 0)