don't let blocks get end lines before their start lines

rdar://72043555

Author: QuietMisdreavus

api_test/main.c

@@ -1004,13 +1004,14 @@ static void source_pos(test_batch_runner *runner) {
     ">    Sure.\n"
     ">\n"
     "> 2. Yes, okay.\n"
-    ">    ![ok](hi \"yes\")\n";
+    ">    ![ok](hi \"yes\")\n"
+    "<!-- HTML Comment -->";
 
   cmark_node *doc = cmark_parse_document(markdown, sizeof(markdown) - 1, CMARK_OPT_DEFAULT);
   char *xml = cmark_render_xml(doc, CMARK_OPT_DEFAULT | CMARK_OPT_SOURCEPOS);
   STR_EQ(runner, xml, "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"
                       "<!DOCTYPE document SYSTEM \"CommonMark.dtd\">\n"
-                      "<document sourcepos=\"1:1-10:20\" xmlns=\"http://commonmark.org/xml/1.0\">\n"
+                      "<document sourcepos=\"1:1-11:21\" xmlns=\"http://commonmark.org/xml/1.0\">\n"
                       "  <heading sourcepos=\"1:1-1:13\" level=\"1\">\n"
                       "    <text sourcepos=\"1:3-1:5\" xml:space=\"preserve\">Hi </text>\n"
                       "    <emph sourcepos=\"1:6-1:12\">\n"
@@ -1052,6 +1053,8 @@ static void source_pos(test_batch_runner *runner) {
                       "      </item>\n"
                       "    </list>\n"
                       "  </block_quote>\n"
+                      "  <html_block sourcepos=\"11:1-11:21\" xml:space=\"preserve\">&lt;!-- HTML Comment --&gt;\n"
+                      "</html_block>\n"
                       "</document>\n",
          "sourcepos are as expected");
   free(xml);

src/blocks.c

@@ -289,7 +289,8 @@ static cmark_node *finalize(cmark_parser *parser, cmark_node *b) {
     b->end_column = parser->last_line_length;
   } else if (S_type(b) == CMARK_NODE_DOCUMENT ||
              (S_type(b) == CMARK_NODE_CODE_BLOCK && b->as.code.fenced) ||
-             (S_type(b) == CMARK_NODE_HEADING && b->as.heading.setext)) {
+             (S_type(b) == CMARK_NODE_HEADING && b->as.heading.setext) ||
+             (parser->line_number - 1 < b->end_line)) {
     b->end_line = parser->line_number;
     b->end_column = parser->curline.size;
     if (b->end_column && parser->curline.ptr[b->end_column - 1] == '\n')