MediaCas : Add missing check to mEventHandler

Test: atest MediaCasTest
Merged-In: Ifd34666b5a1405a26bdde29c62a42feb8b9ed2f5
Change-Id: Ifd34666b5a1405a26bdde29c62a42feb8b9ed2f5
Flag: EXEMPT bugfix
Bug: 371220849
1 file changed