mtd: fix buffer leak and fd leak in mtd_dump()

mtd: fix buffer leak and fd leak in mtd_dump()

Two leaks in mtd_dump():

- The buffer allocated with malloc(erasesize) is never freed before
  returning, leaking erasesize bytes on every call.
- The pre-existing malloc-NULL early return path also leaked the just-
  opened fd by returning directly instead of going through cleanup.

Initialize buf to NULL, route the malloc-NULL case through the
existing 'out:' label, and add free(buf) on the cleanup path so both
fd and buf are released consistently on every exit.

Signed-off-by: Anna Kiri <bredcorn@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23706
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>