clipboard: wrap vstring call in unsafe (#6170)

vlib/clipboard/clipboard_linux.c.v

@@ -309,8 +309,10 @@ fn (mut cb Clipboard) start_listener(){
 						C.XDeleteProperty(event.xselection.display, event.xselection.requestor, event.xselection.property)
 						if cb.is_supported_target(prop.actual_type) {
 							cb.got_text = true
+							unsafe {
 								cb.text = byteptr(prop.data).vstring() //TODO: return byteptr to support other mimetypes
 							}
+						}
 						cb.mutex.unlock()
 					}
 				}