Debugger: Fix CFI length computation.

- Due to a slight oversight during optimization refactoring, the length
  of the FDE's call frame instructions would sometimes be computed
  incorrectly, leading us to overflow past the end of the instructions,
  and fail to unwind the frame correctly if the address in question fell
  at the end. Fixes a regression introduced in commit
  d390ebee9e7355ca364f0e105374a33907a5a7cb.