bittube_fuzzer: Fix memory leak

This test has memory leak, and fuzzing will terminate after a single
iteration. Bittube is a RefBase object, need to use sp instead of
declared it as a local variable directly.

Test: bittube_fuzzer
Bug: 241346940
Bug: 243132994
Change-Id: I34c8fd0a5c5daf8c368689f2104a42e013d3a03f