jffs2: Fix possible null-pointer dereferences in jffs2_add_frag_to_fragtree()

In jffs2_add_frag_to_fragtree(), there is an if statement on line 223 to
check whether "this" is NULL:
    if (this)

When "this" is NULL, it is used at several places, such as on line 249:
    if (this->node)
and on line 260:
    if (newfrag->ofs > this->ofs)

Thus possible null-pointer dereferences may occur.

To fix these bugs, -EINVAL is returned when "this" is NULL.

These bugs are found by a static analysis tool STCheck written by us.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>