Fix ro.build.product not found by ota_from_target_files in some cases

After I5dccba2172dade3dacc55d832a2042fce306b5f5 it was possible that
if override_prop was set and override_device was not set the script
was looking for a prop that did not exist.

Change-Id: I444a33de5bcb59f129bdcf631c2a6540c5926545