shrink if FEATURE_SHADOWPASSWDS is off

diff --git a/libbb/update_passwd.c b/libbb/update_passwd.c
index 35b89a5..aa1e2ed 100644
--- a/libbb/update_passwd.c
+++ b/libbb/update_passwd.c
@@ -100,7 +100,7 @@
 	name = xasprintf("%s:", name);
 	user_len = strlen(name);
 
-	if (strstr(filename, "shadow"))
+	if (ENABLE_FEATURE_SHADOWPASSWDS && strstr(filename, "shadow"))
 		old_fp = fopen(filename, "r+");
 	else
 		old_fp = fopen_or_warn(filename, "r+");