imsm: fix a -O2 build warning

imsm: fix a -O2 build warning

super-intel.c: In function ‘imsm_add_spare’:
super-intel.c:4833: error: ‘array_start’ may be used uninitialized in this function
super-intel.c:4834: error: ‘array_end’ may be used uninitialized in this function

This is valid, if we don't find a spare candidate then array_{start,end}
will be uninitialized.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>