Add a missing if check for flag waypoint health

Fixes #2914 (closed)

The waypoint usage here was missing a check that it has been initialised properly. Without this the waypoint updating code assumed that the flag has a max_health which is non-zero and above zero. This assumption caused many division by zero errors.

This patch only adds the following if check as it is used to initialise the health values:

autocvar_g_ctf_flag_return_time || (autocvar_g_ctf_flag_return_damage && autocvar_g_ctf_flag_health)