Fix exception when delete an uninitialized object

Due to some reason, for example, mediaserver dies, SniffAMR fails and
the variable mMeta is not initialized. Then exception happens when
the flow deletes mMeta in ~AMRExtractor().

Need to check whether mMeta is uninitialized before deleting it.

Bug: 138694976
Test: MTBF test
Change-Id: I398f1d8639a11bf076e51e9d9077a1f2dd89ab52