Fixed HTML comment scanning.

Need to handle this case: `<!--> and -->`.

Since the scanner finds the longest match, we had to
move some of the logic outside of the scanner.

Author: jgm

src/inlines.c

@@ -909,10 +909,17 @@ static cmark_node *handle_pointy_brace(subject *subj, int options) {
     int c = subj->input.data[subj->pos];
     if (c == '!') {
       c = subj->input.data[subj->pos+1];
-      if (c == '-') {
-        matchlen = scan_html_comment(&subj->input, subj->pos + 1);
-        if (matchlen > 0)
-          matchlen += 1; // prefix "<!"
+      if (c == '-' && subj->input.data[subj->pos+2] == '-') {
+	if (subj->input.data[subj->pos+3] == '>') {
+	  matchlen = 4;
+	} else if (subj->input.data[subj->pos+3] == '-' &&
+                   subj->input.data[subj->pos+4] == '>') {
+          matchlen = 5;
+        } else {
+          matchlen = scan_html_comment(&subj->input, subj->pos + 1);
+          if (matchlen > 0)
+            matchlen += 1; // prefix "<"
+	}
       } else if (c == '[') {
         if ((subj->flags & FLAG_SKIP_HTML_CDATA) == 0) {
           matchlen = scan_html_cdata(&subj->input, subj->pos + 2);