0.9.9/client.rb: Catch empty version responses.

Sometimes sphinx will reply with an empty version (for unknown reasons).
The exception raised is a "undefined method `<' for nil:NilClass" which
is both confusing and inconvenient to target for rescuing.

This patch catches such errors and raises a proper Riddle exception
instead.

lib/riddle/0.9.9/client.rb

@@ -12,7 +12,11 @@ def initialise_connection
       socket.send [1].pack('N'), 0
 
       # Checking version
-      version = socket.recv(4).unpack('N*').first
+      unless version = socket.recv(4).unpack('N*').first
+        socket.close
+        raise ResponseError, "No response from searchd when checking version."
+      end
+
       if version < 1
         socket.close
         raise VersionError, "Can only connect to searchd version 1.0 or better, not version #{version}"