BUG/MEDIUM: hlua: Fix integer underflow when receiving line from lua cosocket

BUG/MEDIUM: hlua: Fix integer underflow when receiving line from lua cosocket

In hlua_socket_receive_yield(), when we try to get a line, the trailing CRLF is
stripped by decrementing the block length. The '\n' is first skipped, then,
possible a preceeding '\r'. But the block lenght is never checked. If an empty
line is returned, this leads to an integer underflow and most probably to a
crash because this length is used to copy data into a LUA string.

To fix the issue, the block length is now properly tested against 0 before
decrementing it.

This patch must be backported to all stable versions.