Ensure locked Perfetto handle is non-null.

Although the time between when Trace starts and GetDataSourceLocked is
called is small, it's still possible for the handle to be invalidated
between those two points in time. Check validity prior to use.

Test: atest libnetworkstats_test
bug: 289476598
Change-Id: Ie4b3d65178e6ca24382efde3f38162f332144e81