Recently I learnt that fprintf() is not async-signal-safe. Maybe it also
attempts to allocate memory sometimes. Hence, using it when we
presumably are out of memory is wishful thinking.

Therefore replace that with async-signal-safe code. If you have to check
pointers from traditional signal handlers, now you could do that too!

While doing this, we also lose the string formatting for line number. I
would argue that printing file and line number is not that useful, if
the system really is out of memory. If not out of memory, a core dump
would give us much more detailed information about what went wrong.

clients/window.c had some calls to fail_on_null() and these are simply
replaced. They were used for checking that creating new wl_proxy by
issuing a protocol request worked, and IIRC that only fails on
out-of-memory, so the same rationale applies here.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>