Fix possibility of uninitialized vals in convolve_test
Fix possibility of uninitialized vals in convolve_test

This clears static analysis warnings due to unset values
in the intermediate buffer. This occurs when the intermediate
buffer height is less then the dimensions of the filter.

Change-Id: I73bab842986f5c7a225025e2c9da920694767576

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom