jffs2: Avoid unneeded 'if' before kfree

kfree() deals gracefully with NULL pointers, so it's pointless to test for
one prior to calling it.
This removes such a test from jffs2_scan_medium().

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>