wil6210: check len before memcpy() calls

memcpy() in wmi_set_ie() and wmi_update_ft_ies() is called with
src == NULL and len == 0. This is an undefined behavior. Fix it
by checking "ie_len > 0" before the memcpy() calls.

As suggested by GCC documentation:
"The pointers passed to memmove (and similar functions in <string.h>)
must be non-null even when nbytes==0, so GCC can use that information
to remove the check after the memmove call." [1]

[1] https://gcc.gnu.org/gcc-4.9/porting_to.html



Cc: Maya Erez <merez@codeaurora.org>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: stable@vger.kernel.org
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>