common/obj_bencher: Fix error return check from read that is negative on error

Fixed read return value in d99f1d9f68db41231e0ffff4082b05d6d095c231

Fixes: #8100

Signed-off-by: David Zafman <david.zafman@inktank.com>
Merge branch 'wip-8100'

Reviewed-by: Mark Nelson <mark.nelson@inktank.com>