include/cpp-btree: fix array bounds warning in child() accessors

include/cpp-btree: fix array bounds warning in child() accessors

Replace array indexing with pointer arithmetic in child() and
mutable_child() methods to avoid compiler warning when accessing
child nodes beyond the static array bounds.

The original code was functionally correct but triggered
-Warray-bounds when accessing mutable_child(32) during btree
operations. Using pointer arithmetic achieves the same result
without the bounds check warning.

This is a follow-up to commit 8458a19ab which fixed similar
warnings in other btree_node methods.

No functional changes.

Fixes: https://tracker.ceph.com/issues/72477
Signed-off-by: Kefu Chai <tchaikov@gmail.com>