disk_cache: Fix filename leak on error path.

disk_cache: Fix filename leak on error path.

Remove filename ralloc comment. filename is allocated by asprintf.

Clean up disk_cache_get dead code left over from 367ac07efcc8
("disk_cache: move cache item loading code into
disk_cache_load_item() helper").

Fix defect reported by Coverity Scan.
Logically dead code (DEADCODE)
dead_error_line: Execution cannot reach this statement:
free(filename);

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6738>