[DynInst_API:] [dyninst/dyninst] 64ddbf: fix Instruction class to allow valid assignment

[kupsch <noreply@xxxxxxxxxx>]

  Branch: refs/heads/kupsch/fix-instruction-class-formatter
  Home:   https://github.com/dyninst/dyninst
  Commit: 64ddbfa954d11a24836f43bf34da82e7b58d8b2c
      https://github.com/dyninst/dyninst/commit/64ddbfa954d11a24836f43bf34da82e7b58d8b2c
  Author: James A. Kupsch <kupsch@xxxxxxxxxxx>
  Date:   2022-11-14 (Mon, 14 Nov 2022)

  Changed paths:
    M instructionAPI/h/Instruction.h
    M instructionAPI/src/Instruction.C

  Log Message:
  -----------
  fix Instruction class to allow valid assignment

* Allow the Instruction class to allow correct assignment semantics by
  changing a member from a reference to a pointer

* The formatter member is a reference (to a singleton) that can not be
  changed after it is initialized.  Assignment to this member wrongly
  assigns to the previously bound referent singleton and wrongly slices
  the rhs to the ArchSpecificFormatter base class.  The base class has
  no data members, and since the assignment does not change the type of
  the object (its vtable), it appears to have no effect.  Also a default
  constructed Instuction object sets the formatter to the x86 formatter
  even if the host was not x86.

* Change Instruction::format to return an error string if the arch is
  set to Arch_none (default constructed, to prevent nullptr dereference)