gpu: ion: add pointer null checks in map and unmap

Clients may pass incorrect arguments to ION causing crashes.
Add checks to validate input for ION APIs.

Change-Id: Idafa4535b3ef71184ca11fb2c0c4ab073a52ac2f
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>