BUG: Fix uninitialized variable in GradientRecursiveGaussianImageFilter

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

Change-Id: Ia2a70086b83958320f7e159658ada7df98a753b2
COMP: fix unused variable warning in abort test

Change-Id: I3f36a2ebfb93a7a16f095d948d4579a432fb18e3