Fix undefined behaviour in collection_compare_cb function.

The collection_compare_cb function set the user_data pointer to point to a local variable
of the function.  That pointer is then used in the (only) calling function
qof_collection_compare to make decisions.  This is undefined.  It is likely not an actual
problem because the stack depth of the qof_collection_foreach followed by
collection_compare_cb is deeper than the qof_collection_get_data call (and others
in between if any) that the stack data user data stays uncorrupted.

But, it is undefined behavior, and could cause really subtle bugs if these there are code
changes that have deeper stack between the setting and using.

Also, using this local variable is not necessary, the qof_collection_compare function
already  sets up a variable local to its scope for this that the collection_compare_cb can
use directly.

This commit removes the local (to collection_compare_cb) variable and uses the one setup
in qof_collection_compare.

The full coverage test for qofid.cpp passed before and after this change.

libgnucash/engine/qofid.cpp

@@ -139,7 +139,7 @@ collection_compare_cb (QofInstance *ent, gpointer user_data)
     QofCollection *target;
     QofInstance *e;
     const GncGUID *guid;
-    gint value;
+    gint* value;
 
     e = NULL;
     target = (QofCollection*)user_data;
@@ -147,28 +147,25 @@ collection_compare_cb (QofInstance *ent, gpointer user_data)
     {
         return;
     }
-    value = *(gint*)qof_collection_get_data(target);
-    if (value != 0)
+    value = (gint*)qof_collection_get_data(target);
+    if (*value != 0)
     {
         return;
     }
     guid = qof_instance_get_guid(ent);
     if (guid_equal(guid, guid_null()))
     {
-        value = -1;
-        qof_collection_set_data(target, &value);
+        *value = -1;
         return;
     }
     g_return_if_fail (target->e_type == ent->e_type);
     e = qof_collection_lookup_entity(target, guid);
     if ( e == NULL )
     {
-        value = 1;
-        qof_collection_set_data(target, &value);
+        *value = 1;
         return;
     }
-    value = 0;
-    qof_collection_set_data(target, &value);
+    *value = 0;
 }
 
 gint