ircd: chmode: Avoid referencing beyond the end of the flags_list array in set_channel...

ircd: chmode: Avoid referencing beyond the end of the flags_list array in set_channel_mode

We're setting flags to flags_list[3] at the end of the loop, but the
array only has 3 elements. Unless the compiler optimises this away
(because flags will not be used again) we're accessing memory beyond
the end of the array.

With gcc-4.9:
chmode.c: In function 'set_channel_mode':
chmode.c:1548:54: warning: iteration 2u invokes undefined behavior [-Waggressive-loop-optimizations]
  for(j = 0, flags = flags_list[0]; j < 3; j++, flags = flags_list[j])
                                                      ^
chmode.c:1548:2: note: containing loop
  for(j = 0, flags = flags_list[0]; j < 3; j++, flags = flags_list[j])

Explicitly set "flags = flags_list[j]" at the start of each loop
iteration, which will avoid referencing off the end of the array.