bug#21338: 25.0.50; 25.0.50; forward-symbol works improperly for Elisp ?\(

[Oleh Krehel]

For example, starting with an Elisp buffer "|?\(", where "|" is the
point, I would expect (forward-symbol 1) to change the buffer to "?\(|".
Instead, it ends up as "?|\(".

This also affects the result of (bounds-of-thing-at-point 'symbol) for
the above construct. Note that `forward-symbol' works correctly when
there is no backslash, e.g. for "|?a".