char_superscript incorrectly handles backslashes

[jercos]

Currently, on a superscript grouping ^(like this), backslashes cause an instant end to the group, as though the first backslash seen was a closing paren. This behavior seems undesirable, and most likely unintended.

Example: ^((\^test\)) should result in <p><sup>(^test)</sup></p>\n, but actually results in <p><sup>(\</sup>test))</p>