SLOs created after commit 2d25fe6ad3573b2a06b6b3e5e66493d7b0c55693
have an explicit X-Object-Sysmeta-SLO-Size attribute. Fetching one of
those with a range like "bytes=100-" would crash and cause a 500. This
commit fixes the crash.
Closes-Bug: #1656147
Change-Id: I174ed98b4f01a5943af12177d6f8c3586f049778