mtd: check malloc() return value in mtd_check()

mtd: check malloc() return value in mtd_check()

After malloc(erasesize) in mtd_check(), the result was never checked
for NULL. On allocation failure, the buf pointer remained NULL and
would later be used in image_check() via read(imagefd, buf + buflen,
...), causing a NULL pointer dereference.

Add a NULL check after the allocation and return early. The early
return path also closes the just-opened fd and frees the strdup'd
colon-separated device-list copy to avoid leaks.

Signed-off-by: Anna Kiri <bredcorn@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23705
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>