instrIndex in the current UIActionContext becomes -1 after navigating to a history entry in the linear view

[xusheng6]

This is created to track the leftover issue of #6254. Basically, since we are not restoring the instrIndex when we navigate to a history entry in the linear view, it gets its default value (-1). While this behavior is much better than the original behavior (the value is read before initialization and can return anything), it is still not the most desired result.

We cannot simply save the instrIndex into the HistoryEntry object -- since the instrIndex could change in the future. Also, this issue is unlikely to make substantial difference to the user -- since if they randomly click somewhere in the linear view, these things will all be populated correctly. In other words, the instrIndex is only missing when the user does not click anywhere else. In this sense, if we do not find out a good way to restore it, I think it is tolerable to leave it as-is. That said, I am still creating this issue to track the progress and latest status on it

[xusheng6]

Repro steps:

1. Open any binary and switch to linear view
2. Double-click a function to navigate to it
3. Press Escape to go back
4. Run following code in Python console:

>>> hex(current_ui_action_context.instrIndex)
'0xffffffffffffffff'

5. Also notice if I click on any other instruction, the value becomes correctly populated:

>>> hex(current_ui_action_context.instrIndex)
'0x4'

[xusheng6]

After extensive discussion, we decide the fix for this would be to NOT call the callback if the instrIndex is -1