ext4: drop an always true check

EXT_FIRST_INDEX(ptr) is ptr+12, which can't possibly be null; gcc-12
warns about this.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/20211115172020.57853-1-kilobyte@angband.pl


Signed-off-by: Theodore Ts'o <tytso@mit.edu>