timers/migration: Prevent out of bounds access on failure

timers/migration: Prevent out of bounds access on failure

When tmigr_setup_groups() fails the level 0 group allocation, then the
cleanup derefences index -1 of the local stack array.

Prevent this by checking the loop condition first.

Fixes: 7ee988770326 ("timers: Implement the hierarchical pull model")
Signed-off-by: Levi Yun <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Anna-Maria Behnsen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]