UBI: fix error path in __wl_get_peb

In case of an error (if there are not free PEB's for example),
__wl_get_peb will return a negative value. In order to prevent access
violation we need to test the returned value prior to using it later on.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Reviewed-by: Dolev Raviv <draviv@codeaurora.org>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>