check that a valid pipe is passed before doing a pipe close.

check that a valid pipe is passed before doing a pipe close.

I made this change after getting a segv in reply_pipe_close(). The
funny thing was that pipes_open was 1 and Pipes was NULL. That "can't
happen" and suggests that we have a wild pointer somewhere.

I suspect the rpc code, as I was playing with long share names (a
share called "averylongusername") at the time and the logs show lots
of srvsvc operations. I bet there is a buffer in the rpc code
somewhere that is overflowing and trashing bits of the data segment.