diff: fix textconv error zombies

To make the code simpler, run_textconv lumps all of its
error checking into one conditional. However, the
short-circuit means that an error in reading will prevent us
from calling finish_command, leaving a zombie child.
Clean up properly after errors.

Based-on-work-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -3865,6 +3865,7 @@ static char *run_textconv(const char *pgm, struct diff_filespec *spec,
 	const char **arg = argv;
 	struct child_process child;
 	struct strbuf buf = STRBUF_INIT;
+	int err = 0;
 
 	temp = prepare_temp_file(spec->path, spec);
 	*arg++ = pgm;
@@ -3875,16 +3876,20 @@ static char *run_textconv(const char *pgm, struct diff_filespec *spec,
 	child.use_shell = 1;
 	child.argv = argv;
 	child.out = -1;
-	if (start_command(&child) != 0 ||
-	    strbuf_read(&buf, child.out, 0) < 0 ||
-	    finish_command(&child) != 0) {
-		close(child.out);
-		strbuf_release(&buf);
+	if (start_command(&child)) {
 		remove_tempfile();
-		error("error running textconv command '%s'", pgm);
 		return NULL;
 	}
+
+	if (strbuf_read(&buf, child.out, 0) < 0)
+		err = error("error reading from textconv command '%s'", pgm);
 	close(child.out);
+
+	if (finish_command(&child) || err) {
+		strbuf_release(&buf);
+		remove_tempfile();
+		return NULL;
+	}
 	remove_tempfile();
 
 	return strbuf_detach(&buf, outsize);