iw: scan: fix buffer over-read in print_ies()

iw: scan: fix buffer over-read in print_ies()

This patch correctly checks, if enough data bytes for parsing IEs are
present (-2 in check for type and length). Furthermore, it adds a
nullptr and length check to ease future fuzzing.

Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Link: https://lore.kernel.org/r/20200209165902.44110-3-markus.theil@tu-ilmenau.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>