Crashfix: PART

[michaelortmann]

Found by: BigBadWouf
Patch by: michaelortmann
Fixes: #1708

One-line summary:
Crashfix gotpart(): If ismember() returns 0, fallback to get_user_by_host()

Additional description (if needed):
ismember() can returns 0 while the bot has not (yet) synched the channel, like when it just joined
This bug affects eggdrop version 1.10.0, older versions are not affected

Test cases demonstrating functionality (if applicable):
Fixes the following crash:

[22:08:48] [@] :testuser!~michael@localhost PART #test666
[22:08:48] * Please report problem to https://github.com/eggheads/eggdrop/issues
[22:08:48] * Check doc/BUG-REPORT on how to do so.
[22:08:48] * Last bind (may not be related): evnt:init_server
[22:08:48] * Wrote DEBUG
[22:08:48] * SEGMENT VIOLATION -- CRASHING!
Segmentation fault (core dumped)

[BigBadWouf]

Hi,

Thank you, this perfectly fix the /part crash. However it's seem produce a lot call to killmember(). Don't know if related

[michaelortmann]

Good find. The killmember() was already called before the regression.

So now, additionally this PR avoids calling killmember() if we just checked that the parting entity is not ismember()

ismember() already checks the very channel memberlist that killmember() would search through again.

[thommey]

Good fix, just add that TODO comment please whenever we use straight get_user_from_host because we want to look at those instances again in the future to work with a possible account-tag we might've gotten (needs to change raw bind to rawt etc.), not part of this PR, just as a comment, we have similar ones in other places already.