staging: android: ram_console: fix crash in ram_console_late_init

If the persistent ram buffer is not available, ram_console_late_init
would crash when dereferencing ram_console_zone.  Return early if
ram console was not initialized.

CC: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Change-Id: I06c9c3b28a1249441fa3aa6897c1ba2510613538