fix: panics when negative indent with tabs in fenced code block. Fixes #522

Author: yuin

_test/extra.txt

@@ -861,3 +861,18 @@ text" /></p>
 <li>test2</li>
 </ul>
 //= = = = = = = = = = = = = = = = = = = = = = = =//
+
+69: Negative indentation with tabs in fenced code block
+    OPTIONS: {"enableEscape": true}
+//- - - - - - - - -//
+*
+\t ~~~
+\t0
+//- - - - - - - - -//
+<ul>
+<li>
+<pre><code>0
+</code></pre>
+</li>
+</ul>
+//= = = = = = = = = = = = = = = = = = = = = = = =//

parser/fcode_block.go

@@ -89,10 +89,7 @@ func (b *fencedCodeBlockParser) Continue(node ast.Node, reader text.Reader, pc C
 	}
 	pos, padding := util.IndentPositionPadding(line, reader.LineOffset(), segment.Padding, fdata.indent)
 	if pos < 0 {
-		pos = util.FirstNonSpacePosition(line)
-		if pos < 0 {
-			pos = 0
-		}
+		pos = max(0, util.FirstNonSpacePosition(line)) - segment.Padding
 		padding = 0
 	}
 	seg := text.NewSegmentPadding(segment.Start+pos, segment.Stop, padding)