iwlwifi: fix kernel oops when ucode DMA memory allocation failure

The patch fixes memcpy to NULL address when the ucode DMA allocation failure.

This is a fix to bug
http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1861



Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>