fix: #568

Author: yuin

_test/extra.txt

@@ -883,3 +883,18 @@ text" /></p>
 //- - - - - - - - -//
 <h1>A</h1>
 //= = = = = = = = = = = = = = = = = = = = = = = =//
+
+71: Negative indentation with tabs in fenced code block
+    OPTIONS: {"enableEscape": true}
+//- - - - - - - - -//
+*
+\t* \t~
+//- - - - - - - - -//
+<ul>
+<li>
+<ul>
+<li>~</li>
+</ul>
+</li>
+</ul>
+//= = = = = = = = = = = = = = = = = = = = = = = =//

parser/fcode_block.go

@@ -35,6 +35,9 @@ func (b *fencedCodeBlockParser) Trigger() []byte {
 func (b *fencedCodeBlockParser) Open(parent ast.Node, reader text.Reader, pc Context) (ast.Node, State) {
 	line, segment := reader.PeekLine()
 	pos := pc.BlockOffset()
+	if pos < 0 || (line[pos] != '`' && line[pos] != '~') {
+		return nil, NoChildren
+	}
 	findent := pos
 	fenceChar := line[pos]
 	i := pos