include/cpp-btree: fix false -Warray-bounds in child accessors

include/cpp-btree: fix false -Warray-bounds in child accessors

After inlining, GCC's VRP sees mutable_child() reaching a leaf-root
node whose static type only bounds values[], not children[], and fires
even though the if(!leaf()) guard prevents it at runtime:

  btree.h:522: warning: array subscript [33, 287] is outside array
  bounds of 'struct M[32]' [-Warray-bounds]

Decay children[] to a raw pointer in child()/mutable_child() so GCC
has no array bounds to check.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>