snprintf with overlapping source and dest strings

[michaelortmann]

https://github.com/eggheads/eggdrop/blob/6e9aeacf78fe2b869d73e2c07819619415c1cb25/src/mod/server.mod/server.c#L586

snprintf with overlapping source and dest strings is undefined behaviour, dst is char * restrict.

After inserting debug code i was able to see a real bug here.

I found this with #1028, but Will Buckner also found this before me in 2005:

http://lists.eggheads.org/pipermail/eggdev/2005-April/015490.html

He mentioned a patch, but i checked eggdrops commit history and this patch never made it into eggdrop.

My attempt to contact Mr. Buckner failed due to his old email address being not valid anymore.