clients: fix errno handling

clients/editor.c: In function ‘read_file’:
clients/editor.c:1578:16: warning: logical ‘or’ applied to non-boolean
constant [-Wlogical-op]
  errno = errsv || EINVAL;

This works in the shell, but not in C. Introduced in 411ffabbb5

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Emilio Pozuelo Monfort <pochu@debian.org>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

clients/editor.c

@@ -1575,7 +1575,7 @@ error:
 	if (fin)
 		fclose(fin);
 	free(buffer);
-	errno = errsv || EINVAL;
+	errno = errsv ? errsv : EINVAL;
 
 	return NULL;
 }