Check 'ix' before it is used.

Fixes: 80e675f9 ("ext4: optimize memmmove lengths in extent/index insertions")
Signed-off-by: Gou Hao <gouhao@uniontech.com>
Link: https://lore.kernel.org/r/20230906013341.7199-1-gouhao@uniontech.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>