libstagefright: Check for duration > 0 to avoid DivideByZero crash

- duration = 0 can cause divide by zero and for this clip duration
  is indiacted as 0.
- check for duration > 0 rather than duration >= 0

Change-Id: I58ccacbf7ede892dff9626715162ea7b1f2ddbc6
CRs-Fixed: 451855