minor fix on debug mode to prevent crashing

Session/Conversations/ConversationViewItem.m

@@ -569,13 +569,13 @@ NSString *NSStringForOWSMessageCellType(OWSMessageCellType cellType)
             TSAttachment *_Nullable linkPreviewAttachment =
                 [TSAttachment fetchObjectWithUniqueID:message.linkPreview.imageAttachmentId transaction:transaction];
             if (!linkPreviewAttachment) {
-                OWSFailDebug(@"Could not load link preview image attachment.");
+                OWSLogDebug(@"Could not load link preview image attachment.");
             } else if (!linkPreviewAttachment.isImage) {
-                OWSFailDebug(@"Link preview attachment isn't an image.");
+                OWSLogDebug(@"Link preview attachment isn't an image.");
             } else if ([linkPreviewAttachment isKindOfClass:[TSAttachmentStream class]]) {
                 TSAttachmentStream *attachmentStream = (TSAttachmentStream *)linkPreviewAttachment;
                 if (!attachmentStream.isValidImage) {
-                    OWSFailDebug(@"Link preview image attachment isn't valid.");
+                    OWSLogDebug(@"Link preview image attachment isn't valid.");
                 } else {
                     self.linkPreviewAttachment = linkPreviewAttachment;
                 }