fix handling of unusual exception handler layout

Scala sometimes generates bytecode such that the scope of an exception
handler starts at another exception handler, e.g.:

      Exception table:
         from    to  target type
           290   372   382   any
           382   451   451   any
           290   372   451   any

Avian's compiler was incorrectly initializing the stack frame for the
second handler in this case.  This commit fixes the problem.