gdb/python: check return from final PyObject_New in py-disasm.c

In this commit:

  commit dbd05b9edcf760a7001985f89bc760358a3c19d7
  Date:   Wed Aug 20 10:45:09 2025 +0100

      gdb/python: check return value of PyObject_New in all cases

I missed a call to PyObject_New in python/py-disasm.c, which this
commit addresses.

Unlike the previous commit, the call to PyObject_New in py-disasm.c is
contained within the scoped_disasm_info_object class, which makes it
harder to check for NULL and return.

So in this commit I've rewritten the scoped_disasm_info_object class,
moving the call to PyObject_New out into gdbpy_print_insn, which is
the only place that scoped_disasm_info_object was being used.

As scoped_disasm_info_object is no longer responsible for creating the
underlying Python object, I figured that I might as well move the
initialisation of that object out of scoped_disasm_info_object too.

With that done, the scoped_disasm_info_object now has just one task,
invalidating the existing disasm_info_object at the end of the scope.

So I renamed scoped_disasm_info_object to
scoped_invalidate_disasm_info, which reflects its only task.

I made a couple of other small adjustments that were requested during
review, these are both in the same code area: updating
disasm_info_fill to take an object reference rather than a pointer,
and removing the local variable insn_disas_obj from gdbpy_print_insn,
and inline its value at the one place it was used.

There should be no user visible changes after this commit. Except for
the PyObject_New call, which now has proper error checking.  But in
the working case, nothing should have changed.

Approved-By: Simon Marchi <simon.marchi@efficios.com>