s2s tracebacks when connection creation fails

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One way to reproduce this:

1. Run Prosody using select()
2. Have 1024 connections open
3. Try to establish a connection to a remote server

What is the expected output? What do you see instead?

Dec 31 14:16:34 mod_c2s error   Traceback[c2s]: 
/usr/lib/prosody/net/adns.lua:43: /usr/lib/prosody/net/adns.lua:43: 
/usr/lib/prosody/modules/mod_s2s/s2sout.lib.lua:300: attempt to index local 
'conn' (a nil value)
stack traceback:
        [C]: ?
        /usr/lib/prosody/net/adns.lua:43: in function 'lookup'
        /usr/lib/prosody/modules/mod_s2s/s2sout.lib.lua:87: in function 'attempt_connection'
        /usr/lib/prosody/modules/mod_s2s/s2sout.lib.lua:52: in function 'initiate_connection'
        /usr/lib/prosody/modules/mod_s2s/mod_s2s.lua:129: in function '?'
        /usr/lib/prosody/util/events.lua:67: in function 'fire_event'
        /usr/lib/prosody/core/stanza_router.lua:214: in function 'core_route_stanza'
        /usr/lib/prosody/core/stanza_router.lua:191: in function 'core_post_stanza'
        /usr/lib/prosody/modules/mod_presence.lua:89: in function 'handle_normal_presence'
        /usr/lib/prosody/modules/mod_presence.lua:325: in function '?'
        ...
        [C]: in function 'parse'
        /usr/lib/prosody/util/xmppstream.lua:255: in function 'feed'
        /usr/lib/prosody/modules/mod_c2s.lua:230: in function 'data'
        /usr/lib/prosody/modules/mod_c2s.lua:252: in function </usr/lib/prosody/modules/mod_c2s.lua:249>
        (tail call): ?
        /usr/lib/prosody/net/server_select.lua:854: in function </usr/lib/prosody/net/server_select.lua:836>
        [C]: in function 'xpcall'
        /usr/bin/prosody:376: in function 'loop'
        /usr/bin/prosody:407: in main chunk
        [C]: ?

Original issue reported on code.google.com by MWild1 on 4 Jan 2015 at 8:52