stagefright: avoid crash while seeking MKV content

by returning from seek and avoid a NULL pointer access
if the cue-point for video cannot be found at intended
seek position.

CRs-Fixed: 681515
Change-Id: I4d60a1dcf70d162031a3df70ec2847d02dbc7ab1
Merge "stagefright: avoid crash while seeking MKV content" into du44