Blender Git Loki

Fix XR action map index initialization

This reverts 151eed752b01. Originally thought it was necessary to
initialize selected/active indices to -1 to prevent out-of-bounds
list access, but this is not needed since null checks are already
performed after obtaining list members via BLI_findlink().

In addition, leaving indices zero-initialized facilitates use of the
Python API, for example when displaying action map information in a
UI list.