ctags: Error out if writing to stdout failed.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	cracauer, allanjude
Differential Revision:	https://reviews.freebsd.org/D40200

usr.bin/ctags/ctags.c

@@ -218,6 +218,8 @@ main(int argc, char **argv)
 			}
 		}
 	}
+	if (ferror(stdout) != 0 || fflush(stdout) != 0)
+		err(1, "stdout");
 	exit(exit_val);
 }