fix hang when multiple underscores in words

Issue #387
Fixes the hang related to running these examples. 
```
marked('_Result\r\n__________Calculate\r\n__________TempFile\r\n__________tmpForCompareFile TransFile\r\n__________datFile\r\n__________2\r\n')

marked('_a__________b__________c__________d__________e__________f')
```