--bug=fix bug:index out of range panic in v2.listItem (line 1369 in b…

[chp0304]

Fix: Index out of range panic in listItem function

Problem

The listItem function in block.go can cause a panic with runtime error: index out of range when parsing markdown list items that don't end with a newline character.

Stack trace:

panic: runtime error: index out of range [329] with length 329
github.com/russross/blackfriday/v2.(*Markdown).listItem(...)
    block.go:1368

Root Cause

Several loops in the listItem function lack proper bounds checking:

1. Line 1129-1131: Loop skipping whitespace doesn't check i < len(data)
2. Line 1135-1137: Loop finding newline doesn't check bounds, and data[i-1] causes index -1 access when i=0
3. Line 1160-1162: Similar missing bounds check when finding line endings
4. Line 1253-1255: Loop in definition list handling lacks bounds check
5. dliPrefix function: Accesses data[i+1] without checking array length

Fix

Added proper bounds checking to all affected loops:

// Before
for data[i] == ' ' { i++ }

// After  
for i < len(data) && data[i] == ' ' { i++ }

// Before
for i > 0 && data[i-1] != '\n' { i++ }

// After
for i < len(data) && (i == 0 || data[i-1] != '\n') { i++ }

Changes

• block.go: Added bounds checking in listItem function (4 locations)
• block.go: Added length check in dliPrefix function
• block_test.go: Added TestListItemBoundsCheck with 9 test cases covering edge cases

Test Cases

The new test covers:

• Unordered/ordered lists without trailing newline
• Lists with excessive spaces
• Nested lists without trailing newline
• Definition lists without trailing newline
• Empty list markers
• Lists with only spaces after marker

All existing tests pass.