restripe: fix ignoring return value of ‘read’ and lseek

Got below error when run "make everything".

restripe.c: In function ‘test_stripes’:
restripe.c:870:4: error: ignoring return value of ‘read’, declared with attribute warn_unused_result [-Werror=unused-result]
    read(source[i], stripes[i], chunk_size);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix it by check the return value of ‘read’, and free memory
in the failure case.

And check the return value of lseek as well per Jes's comment.

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>