disk_cache: Fix issue reading GLSL metadata

disk_cache: Fix issue reading GLSL metadata

This would cause the read of the metadata content to fail, which would
prevent the linking from being skipped.

Seen on Rocket League with i965 shader cache.

Fixes: b86ecea3446e "util/disk_cache: write cache item metadata to disk"
Cc: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>