Bail if read_nonblock returns nil. Fixes #1502

The code to perform the wait checking incorrectly assumed that data
would return a symbol OR a String. It also returns nil if the socket has
been closed. We must therefore deal with that nil by returning nothing.

lib/puma/minissl.rb

@@ -58,6 +58,8 @@ module Puma
               else
                 IO.select(nil, [@socket.to_io])
               end
+            elsif !data
+              return nil
             else
               break
             end