staging: wilc1000: add check for kmalloc allocation failure.

[ Upstream commit 6cc0c259d034c6ab48f4e12f505213988e73d380 ]

Add a sanity check that wid.val has been allocated, fixes a null
pointer deference on stamac when calling ether_add_copy.

Detected by CoverityScan, CID#1369537 ("Dereference null return value")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>