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
diff --git a/test/convolve_test.cc b/test/convolve_test.cc
index 760b469..a84ef4e 100644
--- a/test/convolve_test.cc
+++ b/test/convolve_test.cc
@@ -106,6 +106,8 @@
       (kInterp_Extend - 1) + output_height + kInterp_Extend;
   unsigned int i, j;
 
+  assert(intermediate_height > 7);
+
   // Size of intermediate_buffer is max_intermediate_height * filter_max_width,
   // where max_intermediate_height = (kInterp_Extend - 1) + filter_max_height
   //                                 + kInterp_Extend