lib/bootconfig: validate child node index in xbc_verify_tree()

lib/bootconfig: validate child node index in xbc_verify_tree()

xbc_verify_tree() validates that each node's next index is within
bounds, but does not check the child index.  Add the same bounds
check for the child field.

Without this check, a corrupt bootconfig that passes next-index
validation could still trigger an out-of-bounds memory access via an
invalid child index when xbc_node_get_child() is called during tree
traversal at boot time.

Link: https://lore.kernel.org/all/20260318155919.78168-9-objecting@objecting.org/
Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>