While it is currently OK to read the global variables backing functions
like g_get_user_data_dir() without the g_utils_global lock held (since
such a read is always preceeded by a critical section where the variable
is set to its final value), upcoming changes will allow those variables
to be changed. If they are changed from one thread while another thread
is calling (for example) g_get_user_data_dir(), the final read from the
second thread could race with the first thread.

Avoid that by only reading the global variables with the lock held.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

#538