After free()ing clientData, set it to NULL

We will change rfbClientCleanup() to free the data.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Address #12 ClientData does not get freed

rfbClientSetClientData() allocates a new rfbClientData, but never gets
cleaned up, which causes memory leaks.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore more generated files

While at it, also ignore the documentation of the RFB protocol best
downloaded manually from

	http://www.realvnc.com/docs/rfbproto.pdf

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>