mtd: jffs2: add missing malloc NULL check

mtd: jffs2: add missing malloc NULL check

In mtd_replace_jffs2(), the return value of malloc(erasesize) is never
checked. If the allocation fails, buf remains NULL and the subsequent
memcpy(buf + ofs, ...) in add_data() will dereference NULL, causing a
segfault.

Add a NULL check and return -1 on allocation failure. Match the
diagnostic message used by the sibling mtd_write_jffs2() so the
out-of-memory cause is visible at the call site.

Signed-off-by: Anna Kiri <bredcorn@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23553
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>