staging: rtl8192u: return -ENOMEM on failed allocation of priv->oldaddr

Currently the allocation of priv->oldaddr is not null checked which will
lead to subsequent errors when accessing priv->oldaddr.  Fix this with
a null pointer check and a return of -ENOMEM on allocation failure.

Detected with Coccinelle:
drivers/staging/rtl8192u/r8192U_core.c:1708:2-15: alloc with no test,
possible model on line 1723

Fixes: 8fc8598e ("Staging: Added Realtek rtl8192u driver to staging")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>