Check for unimplemented keymaster function delete_key

delete_key is optional and may not be implemented by
a legacy hal. So the function pointer must be checked for
nullptr prior to being used.

Bug: 36500704
Test: run VTS tests with legacy hal that does not implement delete_key
Change-Id: Ie51dd173314826ef260319153c7df96d88b8a42f
1 file changed