MPEG4Writer: Initialize mime to NULL in addSource

mime was uninitialized before querying from format.
If format doesn't contain mime, then this remained
uninitialized

Test: atest android.mediav2.cts.MuxerUnitTest
Bug: 148916777

Change-Id: I1375dd9e905fe8ee21f6cff9605db0e97114d0fa
1 file changed