multi_runsingle: avoid NULL dereference

When Curl_do() returns failure, the connection pointer could be NULL so
the code path following needs to that that into account.

Bug: http://curl.haxx.se/mail/lib-2013-03/0062.html
Reported by: Eric Hu
curl_multi_wait: avoid second loop if nothing to do

... hopefully this will also make clang-analyzer stop warning on
potentional NULL dereferences (which were false positives anyway).