libril: Fix double freeing of memory in SAP

 service and add null-checks.

The payload of a SAP request could be freed twice in certain scenarios.
Also, add null-checks to prevent dereferencing of null pointers.

Bug: 64729356
Test: Manually run the fuzz tests and ensure that there is no crash in
      rild

Change-Id: Ib7ae269fa5297d6acea267337b220b8858c82bae