(PR#13156) auto_settler_findwork infinite recursion

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=13156 >

I just had a segv caused by seemingly infinite recursion in 
auto_settler_findwork (it overflowed the stack in a related 
function, with a_s_f several thousand invocations deep).
Sadly, I can't repro it from a savegame; it happens 
consistently in a particular server-only game I'm running, 
but it only hits around 1985 so it's kinda slow to test.