btrfs: exit after state insertion failure at btrfs_convert_extent_bit()

If insert_state() state failed it returns an error pointer and we call
extent_io_tree_panic() which will trigger a BUG() call. However if
CONFIG_BUG is disabled, which is an uncommon and exotic scenario, then
we fallthrough and call cache_state() which will dereference the error
pointer, resulting in an invalid memory access.

So jump to the 'out' label after calling extent_io_tree_panic(), it also
makes the code more clear besides dealing with the exotic scenario where
CONFIG_BUG is disabled.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>