gpu: fixup direct copy

Author: atkassen

src/gpu/generic/direct_copy.hpp

@@ -121,7 +121,8 @@ struct direct_copy_t : public primitive_t {
                         || (l_stride == r_stride && l.first > r.first);
             };
             std::sort(blocks.begin() + offset, blocks.end(), cmp);
-            if (offset > 0 && blocks[offset].first == blocks[offset - 1].first
+            if (offset > 0 && blocks.size() > offset
+                    && blocks[offset].first == blocks[offset - 1].first
                     && blocking.strides[blocks[offset].first] == stride) {
                 blocks[offset - 1].second *= blocks[offset].second;
                 stride *= blocks[offset].second;