mruby-socket: fixed raising RuntimeError from #accept; #6554

In mruby, bare `raise` in rescue clause does not re-raise the last
exception. You should explicitly specify the exception.

mrbgems/mruby-socket/mrblib/socket.rb

@@ -280,9 +280,9 @@ class TCPServer < TCPSocket
       TCPSocket._new_with_prelude(fd, "r+") {
         @init_with_fd = true
       }
-    rescue
+    rescue => e
       IO._sysclose(fd) rescue nil
-      raise
+      raise e
     end
   end