select/cray: need to dereference data part

This fixes a copy&paste bug where the wrong memory area was dereferenced,
found by these error messages in the logs:

 [2011-02-01T15:39:08] error: cray/get_select_jobinfo: jobinfo magic bad
 [2011-02-01T15:39:08] error: cray/get_select_jobinfo: jobinfo magic bad
 [2011-02-01T15:39:08] error: orphaned ALPS reservation 1022, trying to remove

While at it, added tests for the return values of these functions (resv_id
may be undefined if the return value is not SLURM_SUCCCESS).

01_Bug-Fix_pointer-dereference.diff