C2SoftCodec: Add nullptr checks in onStop() and onReset() methods

Call to MediaCodec.stop() and MediaCodec.reset() can be made even
if the component is in an uninitialized state. Add checks to avoid
null dereferencing.

Bug: 422986990
Test: atest VtsHalMediaC2V1_0TargetMasterTest
Test: atest VtsHalMediaC2V1_0TargetComponentTest
Test: atest VtsHalMediaC2V1_0TargetAudioDecTest
Test: atest VtsHalMediaC2V1_0TargetAudioEncTest
Test: atest VtsHalMediaC2V1_0TargetVideoDecTest
Test: atest VtsHalMediaC2V1_0TargetVideoEncTest
Test: atest MctsMediaV2TestCases
Flag: EXEMPT bugfix

Change-Id: I01f01ec40b29fdb60fd70e1f80f8031f609e2e45