KEYS: potential uninitialized variable

If __key_link_begin() failed then "edit" would be uninitialized.  I've
added a check to fix that.

Change-Id: I0e28bdba07f645437db2b08daf67ca27f16c6f5c
Fixes: f70e2e06 ('KEYS: Do preallocation for __key_link()')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>