filtering: close all output descriptors, so that 'xsel' will terminate

For some reason, when copying something to the system clipboard
with 'xsel', it wants to see all output descriptors closed before
it will exit without requiring ^C.

This fixes https://savannah.gnu.org/bugs/?62276.
Reported-by: Shi Yanling <sylphenix@126.com>

Bug existed since version 2.9.8, since piping text through an
external command was introduced.

src/files.c

@@ -1034,6 +1034,7 @@ bool execute_command(const char *command)
 		if (should_pipe) {
 			if (dup2(to_fd[0], STDIN_FILENO) < 0)
 				exit(5);
+			close(from_fd[1]);
 			close(to_fd[1]);
 		}