Mark Wutzke points out that the comment in sk_proxy_set_frozen()

Mark Wutzke points out that the comment in sk_proxy_set_frozen()
states that plug_receive() may recurse back into
sk_proxy_set_frozen() again. Therefore, bufchain_consume() should
have been called _before_ calling plug_receive(), to prevent an
infinite loop overflowing the stack. I can't immediately figure out
under what circumstances this might happen, but it seems an
obviously sensible precaution.

[originally from svn r3741]