mFilterCount cannot be negative

mFilterCount is a reference counter for Filters, and it's decremented
when Filter.stop() is called. Added a check to make sure the value
is not decremented when it's already 0.

Bug: 288170590
Test: atest VtsHalTvTunerTargetTest
Change-Id: I72fe7e5c4babd7e7426d8a52b40887a8c411ce81