BWAPIClient: Fixed bug where you could get uninitialized data from inaccessible units (that have never been accessible).

Since unit data is only updated for accessible units, and you can get inaccessible units through eg. getAddon(), it was possible to get references to units whose data had never been initialized. This data would usually be zeroed the first match, but would not be zeroed between matches. The zeroing is most important for making exists() return false.