Although this shouldn't occur in practice, it's a good idea to bounds
check the length field of the SSID element prior to using it for things
like allocations or memcpy operations.

Cc: <stable@vger.kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Reported-by: NNicolas Waisman <nico@semmle.com>
Signed-off-by: NWill Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20191004095132.15777-1-will@kernel.orgSigned-off-by: NJohannes Berg <johannes.berg@intel.com>