Fix mediadrmserver segfault in GTS tests

When clearHeapBase is called after a plugin is
closed, a null pointer is dereferenced. Protect
against it with a null pointer check.

Test: GTS media tests, check logcat for faults

bug:80434750
Change-Id: Ib568694c44fe22887c3db4f88e67cb598fd9e36a
1 file changed