fix string copying error

String terminator for return value is not copied (prop[nitems]='\0').
See http://tronche.com/gui/x/xlib/window-information/XGetWindowProperty.html (XGetWindowProperty() always allocates one **extra** byte in prop_return and sets it to zero).
This fixes displayed server name. Bug does not appear under gdb or with -DDEBUG.
![pasystray](https://f.cloud.github.com/assets/1663640/1320944/219cf902-3388-11e3-996c-5378e8aa4499.png)