RedefineClasses doesn't check for ConstantPool overflow

When merging constant pools, if the number of constant pool entries exceeds what fits in a u2, then you can't encode them in bytecodes.

RedefineClasses append_entry doesn't check for u2 overflow.

This code could check for overflow even though the actual merged constant pool could be smaller:

  // worst case merged constant pool length is old and new combined
  int merge_cp_length = the_class->constants()->length()
        + scratch_class->constants()->length();