ggml: fix ggml_graph_cpy undefined behavior (ggml/943)

ggml/include/ggml.h

@@ -681,8 +681,8 @@ extern "C" {
 
     struct ggml_hash_set {
         size_t size;
-        ggml_bitset_t * used;
-        struct ggml_tensor ** keys;
+        ggml_bitset_t * used;       // whether or not the keys are in use i.e. set
+        struct ggml_tensor ** keys; // actual tensors in the set, keys[i] is only defined if ggml_bitset_get(used, i)
     };
 
     // computation graph

ggml/src/ggml.c

@@ -19130,7 +19130,8 @@ void ggml_graph_cpy(struct ggml_cgraph * src, struct ggml_cgraph * dst) {
     }
 
     for (size_t i = 0; i < src->visited_hash_set.size; ++i) {
-        if (src->visited_hash_set.keys[i]) {
+        // copy all hashset keys (tensors) that are in use
+        if (ggml_bitset_get(src->visited_hash_set.used, i)) {
             ggml_hash_insert(&dst->visited_hash_set, src->visited_hash_set.keys[i]);
         }
     }