The file clipboard delegate needs a base URI to operate on for
systems that are not WIN32. Added that to the context and abort
conversion, if that is not set. (currently not fully implemented)
found by cppcheck
[client/Wayland/wlf_cliprdr.c:143] -> [client/Wayland/wlf_cliprdr.c:158]: (warning) Either the condition 'clipboard' is redundant or there is possible null pointer dereference: clipboard.
[client/Wayland/wlf_cliprdr.c:163] -> [client/Wayland/wlf_cliprdr.c:178]: (warning) Either the condition 'clipboard' is redundant or there is possible null pointer dereference: clipboard.
