UBI: Test return value of __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.

Change-Id: I3551d5cca5c61a265a34e0d11e1e185b19501024
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>