[FIX] sap.m.AssociativeOverflowToolbar: getContent proper override
[FIX] sap.m.AssociativeOverflowToolbar: getContent proper override

Problem: In some case the function retuns undefined values, because some
controls are already destroyed.

Solution: Array is filtered to return existing controls.

Change-Id: I15ac56fa270a5814bb91ca78dfb884c00689e465
BCP: 1970601434

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5