use new htmlParseLookupCommentEnd to find comment ends

Note that the caret in error messages generated during comment parsing
may have moved by one byte.

See guidance provided on incorrectly-closed comments here:

https://html.spec.whatwg.org/multipage/parsing.html#parse-error-incorrectly-closed-comment

HTMLparser.c

@@ -5220,6 +5220,39 @@ htmlParseLookupSequence(htmlParserCtxtPtr ctxt, xmlChar first,
     return (-1);
 }
 
+/**
+ * htmlParseLookupCommentEnd:
+ * @ctxt: an HTML parser context
+ *
+ * Try to find a comment end tag in the input stream
+ * The search includes "-->" as well as WHATWG-recommended incorrectly-closed tags.
+ * (See https://html.spec.whatwg.org/multipage/parsing.html#parse-error-incorrectly-closed-comment)
+ * This function has a side effect of (possibly) incrementing ctxt->checkIndex
+ * to avoid rescanning sequences of bytes, it DOES change the state of the
+ * parser, do not use liberally.
+ * This wraps to htmlParseLookupSequence()
+ *
+ * Returns the index to the current parsing point if the full sequence is available, -1 otherwise.
+ */
+static int
+htmlParseLookupCommentEnd(htmlParserCtxtPtr ctxt)
+{
+    int mark = 0;
+    int cur = CUR_PTR - BASE_PTR;
+
+    while (mark >= 0) {
+	mark = htmlParseLookupSequence(ctxt, '-', '-', 0, 0);
+	if ((mark < 0) ||
+	    (NXT(mark+2) == '>') ||
+	    ((NXT(mark+2) == '!') && (NXT(mark+3) == '>'))) {
+	    return mark;
+	}
+	ctxt->checkIndex = cur + mark + 1;
+    }
+    return mark;
+}
+
+
 /**
  * htmlParseTryOrFinish:
  * @ctxt:  an HTML parser context
@@ -5405,8 +5438,7 @@ htmlParseTryOrFinish(htmlParserCtxtPtr ctxt, int terminate) {
 		cur = in->cur[0];
 	        if ((cur == '<') && (next == '!') &&
 		    (in->cur[2] == '-') && (in->cur[3] == '-')) {
-		    if ((!terminate) &&
+		    if ((!terminate) && (htmlParseLookupCommentEnd(ctxt) < 0))
-		        (htmlParseLookupSequence(ctxt, '-', '-', '>', 0) < 0))
 			goto done;
 #ifdef DEBUG_PUSH
 		    xmlGenericError(xmlGenericErrorContext,
@@ -5466,8 +5498,7 @@ htmlParseTryOrFinish(htmlParserCtxtPtr ctxt, int terminate) {
 		next = in->cur[1];
 		if ((cur == '<') && (next == '!') &&
 		    (in->cur[2] == '-') && (in->cur[3] == '-')) {
-		    if ((!terminate) &&
+		    if ((!terminate) && (htmlParseLookupCommentEnd(ctxt) < 0))
-		        (htmlParseLookupSequence(ctxt, '-', '-', '>', 0) < 0))
 			goto done;
 #ifdef DEBUG_PUSH
 		    xmlGenericError(xmlGenericErrorContext,
@@ -5514,8 +5545,7 @@ htmlParseTryOrFinish(htmlParserCtxtPtr ctxt, int terminate) {
 		next = in->cur[1];
 	        if ((cur == '<') && (next == '!') &&
 		    (in->cur[2] == '-') && (in->cur[3] == '-')) {
-		    if ((!terminate) &&
+		    if ((!terminate) && (htmlParseLookupCommentEnd(ctxt) < 0))
-		        (htmlParseLookupSequence(ctxt, '-', '-', '>', 0) < 0))
 			goto done;
 #ifdef DEBUG_PUSH
 		    xmlGenericError(xmlGenericErrorContext,
@@ -5769,9 +5799,7 @@ htmlParseTryOrFinish(htmlParserCtxtPtr ctxt, int terminate) {
 			htmlParseDocTypeDecl(ctxt);
 		    } else if ((cur == '<') && (next == '!') &&
 			(in->cur[2] == '-') && (in->cur[3] == '-')) {
-			if ((!terminate) &&
+			if ((!terminate) && (htmlParseLookupCommentEnd(ctxt) < 0))
-			    (htmlParseLookupSequence(
-				ctxt, '-', '-', '>', 0) < 0))
 			    goto done;
 #ifdef DEBUG_PUSH
 			xmlGenericError(xmlGenericErrorContext,

result/HTML/758606.html.err

@@ -1,6 +1,6 @@
 ./test/HTML/758606.html:1: HTML parser error : Invalid char in comment 0xC
 <!--<!doctype
-      ^
+       ^
 ./test/HTML/758606.html:2: HTML parser error : Comment not terminated 
 <!--<!doctyp
 

result/HTML/758606_2.html.err

@@ -1,6 +1,6 @@
 ./test/HTML/758606_2.html:1: HTML parser error : Invalid char in comment 0xC
 ‘<!dOctYPE
-  ^
+   ^
 ./test/HTML/758606_2.html:2: HTML parser error : Comment not terminated 
 <!--‘<!dOctYP