gstdebug: fix occasional deadlocks on windows when outputting debug logging

gstdebug: fix occasional deadlocks on windows when outputting debug logging

When outputting debug logs on Windows, some sections are protected
with a non-recursive lock. Turns out though that gst_debug_message_get()
might indirectly, via our printf format extensions, call code which
in turn would try to log something when it can't handle something. If
that happens we end up in gst_debug_log_default() again recursively and
try to again take the lock that's already taken, thus deadlocking.

Format the debug message string outside of the critical section
instead to avoid this.

https://bugzilla.gnome.org/show_bug.cgi?id=784382