MSIM:Add null check to avoid phone app crash

When both subscriptions have incoming calls in alerting state, and
both of them disconnected at remote end at same time, Call can be null
in onDisconnect() due to timing issue. Add null check to avoid phone app
crash.

Change-Id: I5c5448765526cc9b6e856476fa05fe9761c2be0e
CRs-Fixed: 652172