files: before sending data to an external command, decode LF back to NUL

(There is no need to recode the NULs back to LFs because the sending of
the data happens in a separate process, which then simply disappears.)

This fixes https://savannah.gnu.org/bugs/?63106.

Bug existed since version 2.9.8, since filtering a buffer or a region
through an external command was introduced.

src/files.c

@@ -988,7 +988,16 @@ void send_data(const linestruct *line, int fd)
 
 	/* Send each line, except a final empty line. */
 	while (line != NULL && (line->next != NULL || line->data[0] != '\0')) {
-		fprintf(tube, "%s%s", line->data, line->next == NULL ? "" : "\n");
+		size_t length = strlen(line->data);
+
+		recode_LF_to_NUL(line->data);
+
+		if (fwrite(line->data, sizeof(char), length, tube) < length)
+			exit(5);
+
+		if (line->next && putc('\n', tube) == EOF)
+			exit(6);
+
 		line = line->next;
 	}