BUG: Fix uninitialized variable in GradientRecursiveGaussianImageFilter

In GradientRecursiveGaussianImageFilter::TransformOutputPixel, the
correctedGradient variable is not fully initialized until the outer loop
completes.

Change-Id: Ia2a70086b83958320f7e159658ada7df98a753b2
Merge topic 'Uninitialized-GradRecurGausFilter'

fbabe3ed BUG: Fix uninitialized variable in GradientRecursiveGaussianImageFilter