weston: arm SEGV handler earlier

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>