Fix slice out of bound exception for xss rule and Fix logic to handle scenario like…

[gkumarcertinia]

Considering scenario like:

......
<script id="MobileChatter" class="result ..........
...........
...........        // ASIST continue scan in script context which is fine 
..........
</script><script id="Default" class="  .............  // ASIST throw `runtime error: slice out of bound`.  Which is fixed in this PR
..............  // ASIST break script context here. Which is fixed in this PR
.............  
.........

1. Fix runtime error: slice out of range

2. Correct logic of ASIST code so that scanning continue in script/style context when a opening script/style tag found after a closing script/style tag in a same line, like the above given scenario.