Ticket #5857: Don't crash in CheckOther::strPlusChar upon invalid code.

Hi,

This trivial patch fixes the ticket by ensuring we don't dereference 0 in strPlusChar upon invalid code. I've tried to detect that we have a syntax error or at least not setup the token's operand2 pointer in that case, but failed to up to now as it caused regressions. Until I figure out how to do it (if it actually is doable...), thanks to consider merging.

Cheers,
  Simon