sd-dhcp6-client: fix off-by-one error in parsing dhcp6 options
    
    This fixes error in parsing message when the rapid commit option is
    located at the end of the message.
    
    Fixes an issure reported in #24002.
    
    (cherry picked from commit 68870a46b3b5d3d5b51d1c27b4128b8fb5188ecb)