user_apps: fix race condition during initialization

If there is a request to allocate memory before domain 3 has been
inited. then, gdev = hdomain[3].dev

If  another thread tries to apps_dev_init domain 2. the device that it
will be used is the one belonging to domain 3.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>