If ptr[0] is NULL, then strchr may return a pointer to the NULL
terminator for serv->nick_prefixes, making the if statement true, which
then leads to the pointer increment leaving ptr oob. Now we check to
ensure ptr[0] != NULL.
From the Linux manpages for strchr:
The terminating null byte is considered part of the string, so that if c is
specified as '\0', these functions return a pointer to the terminator.
