don't insert poster attribute into source tags

lib/single-file/single-file-core.js

@@ -646,7 +646,13 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 
 		insertVideoPosters() {
 			if (this.options.postersData) {
-				this.doc.querySelectorAll("video[src], video > source[src]").forEach((videoElement, videoIndex) => {
+				this.doc.querySelectorAll("video[src], video > source[src]").forEach((element, videoIndex) => {
+					let videoElement;
+					if (element.tagName == "VIDEO") {
+						videoElement = element;
+					} else {
+						videoElement = element.parentElement;
+					}
 					if (!videoElement.poster && this.options.postersData[videoIndex]) {
 						videoElement.setAttribute("poster", this.options.postersData[videoIndex]);
 					}