brcmfmac: validate user provided data for memdump before copying

brcmfmac: validate user provided data for memdump before copying

In patch "brcmfmac: add support for sysfs initiated coredump", a new
scenario of brcmf_debug_create_memdump was added in which the user of
the function might not necessarily provide prefix data. Hence the
function should not assume the data is always valid and should perform a
check before copying.

Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>