Do not call releaseMemoryResource on ignored slot in NN VTS

For IBurst, a slot value of -1 indicates the slot should be ignored.
However, GeneratedTestHarness still attempts to call
IBurst::releaseMemoryResource on ignored slots. Instead, we should skip
releasing any ignored slots.

Bug: 230103381
Test: mma
Test: VtsHalNeuralnetworksTargetTest
Test: presubmit
Change-Id: I82e538aa0fd9e8ecc077df1c1ceece46a6166e67