fat: use the return value of strlcpy instead of strlen.

src/add-ons/kernel/file_systems/fat/attr.c

@@ -233,11 +233,13 @@ dosfs_read_attr(fs_volume *_vol, fs_vnode *_node, void *_cookie, off_t pos,
 		return EINVAL;
 	}
 
-	if (user_strlcpy(buffer, node->mime + pos, *_length) < B_OK) {
+	ssize_t length = user_strlcpy(buffer, node->mime + pos, *_length);
+	if (length < B_OK) {
 		UNLOCK_VOL(vol);
 		return B_BAD_ADDRESS;
 	}
-	*_length = strlen(buffer) + 1;
+	if (length < *_length)
+		*_length = length + 1;
 
 	UNLOCK_VOL(vol);
 	return 0;