chat-view: fix image message aspect ratio calculation

This fixes a binding loop, but introduces another less fatal one,
since commit d6ed9adf.

Change-Id: I67934b4cdde2efe04e20064fe41191241b0689b9
GitLab: #857

src/app/commoncomponents/DataTransferMessageDelegate.qml

@@ -342,7 +342,7 @@ Loader {
                             sourceSize.width: width
                             sourceSize.height: height
                             source: "file:///" + Body
-                            property real aspectRatio: width / implicitHeight
+                            property real aspectRatio: implicitWidth / implicitHeight
                             property real adjustedWidth: Math.min(maxSize,
                                                                   Math.max(minSize,
                                                                            innerContent.width - senderMargin))