Fix memleak in init_channel()

[michaelortmann]

Found by: michaelortmann (or Mystery-X if this fixes #1545, but unlikely, see #1545 (comment))
Patch by: michaelortmann
Fixes:

One-line summary:
Fix memleak in init_channel()

Additional description (if needed):
init_channel() was only freeing the first element of a linked list
Leak is triggered by the bot joining a channel and by use of init_channel(), like with .reset.
Fixing this leak could fix #1545, but it could- very likely - be a different leak.
We decided to ignore codacity code duplication warning here.

Test cases demonstrating functionality (if applicable):
$ valgrind --leak-check=full --show-leak-kinds=all --verbose ./eggdrop -t BotA.conf
make the bot join a channel and wait for the who reply
.die
the leak is shown like:
==320050== by 0x4868D36: init_channel (tclchan.c:2007)

more visually, one could add
char debug[1024 * 1024];
to the memberlist stuct in src/chan.c
so that every leak would leak 1MB