mac80211: fix failure to check kmalloc return value in key_key_read

I noticed two small issues in mac80211/debugfs_key.c::key_key_read while
reading through the code. Patch below.

The key_key_read() function returns ssize_t and the value that's actually
returned is the return value of simple_read_from_buffer() which also
returns ssize_t, so let's hold the return value in a ssize_t local
variable rather than a int one.

Also, memory is allocated dynamically with kmalloc() which can fail, but
the return value of kmalloc() is not checked, so we may end up operating
on a null pointer further on. So check for a NULL return and bail out with
-ENOMEM in that case.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>