files: when piping, always pipe whatever was cut to the external command

Either the marked region was cut (even when zero characters) or
the entire contents of the buffer were cut.  In both cases this
is what needs to be piped to the command.  That is: the contents
of the cutbuffer.

This fixes https://savannah.gnu.org/bugs/?57931.

Bug existed since version 2.9.8, commit f304b9ae,
when piping text to an external command was introduced.

src/text.c

@@ -1004,7 +1004,7 @@ bool execute_command(const char *command)
 
 		/* Create a separate process for piping the data to the command. */
 		if (fork() == 0) {
-			send_data(cutbuffer != NULL ? cutbuffer : openfile->filetop, to_fd[1]);
+			send_data(cutbuffer, to_fd[1]);
 			exit(0);
 		}