gmessages: make _g_log_abort() do only breakpoints again

Commit e53caad4 makes _g_log_abort() noreturn by calling abort()
unconditionally.

However, it is useful to be able to skip some log_abort() with a
debugger, to reach a point of interest. Revert back to previous
behaviour. Make g_assert_warning() noreturn by calling abort().

https://bugzilla.gnome.org/show_bug.cgi?id=711800
gutf8: use g_try_malloc_n

As recommended by Christian Persch.

https://bugzilla.gnome.org/show_bug.cgi?id=711546