Minor cleanup of code in insdel.c

Minor cleanup of code in insdel.c

* src/insdel.c (del_range_2): Update *_BYTE variables _after_
updating the corresponding character values.  This follows what we
do everywhere else, and allows to put a watchpoint on, say, Z_BYTE
to check consistency between the character and byte counts.  See
bug#72165 for one situation where it is useful.

(cherry picked from commit a90a1ed527e89cf26737b6a123070c55c3118923)