Fix two copy/paste bugs: forbid zero-length buffers to prevent a fatal error

when trying to paste them, found by me, and miscalculation of the start/end
causing random fatal errors when copying in copy-mode, reported by sthen.

ok sthen "put it in" deraadt

cmd-paste-buffer.c

@@ -63,7 +63,7 @@ cmd_paste_buffer_exec(struct cmd *self, struct cmd_ctx *ctx)
 		}
 	}
 
-	if (pb != NULL)
+	if (pb != NULL && *pb->data != '\0')
 		buffer_write(w->active->out, pb->data, strlen(pb->data));
 
 	/* Delete the buffer if -d. */