DO NOT MERGE Prevent out of bound memory access in GetInvInt

In GetInvInt(int) function, malicious content can access memory
 outside of the invCount array. Always bound access to valid
 indices.

Test: see bug for malicious content, decoded with "stagefright -s -a"
Bug: 65025048
Change-Id: I92d4a14519f45d5a329d7f69f21f2aef0a8c6daa
(cherry picked from commit 9fb4261c43a2d15f3b77a7e56470ed6784f83d04)