nits: Make sure format is a constant

If format is not a constant an attacker can, by changing the strerror
output cause a string that looks like a format to be returned possibly
causing a security issue.

Change-Id: I00d953a66ec373a9a9f22d0ff2a653c7f154d6bd
Signed-off-by: Saggi Mizrahi <ficoos@gmail.com>