stagefright aacenc: Avoid a division by zero

In rare cases, the result of normFactor * (*psfbNActiveLines)
could be zero, leading to a division by zero in pow2_xy.

In these cases, normFactor was INT_MIN, and if *psfbNActiveLines
was 2, the product was zero.

normFactor being INT_MIN is a bug in itself though.

Change-Id: Ib31bf02889615a7be3bd1bc028da8a7651a8dbee