Fix crash in comment parsing (fixes #1616)

[simon-paris]

This fixes a crash in parsing sequences of comments like the following:

//
/*
*/

The conditions to trigger the crash are:

1. alternateCommentMode is set to true
2. A single line comment is immediately followed by a block comment
3. The block comment spans at least two lines

The crash happens because isDoubleSlashCommentLine returns true for lines starting with /*. It only checks for one slash because in the first callsite, offset points to the second slash character and the character before it is guaranteed to also be a slash. But this is not true in the second callsite. The result is that it treats the line with /* as a single line comment.

[alexander-fenster]

Thank you!