Ensure that a dereferenced variable isn't NULL.

Found by clang static analyzer.

libgnucash/engine/Transaction.c

@@ -1752,7 +1752,7 @@ xaccTransRollbackEdit (Transaction *trans)
         if (!qof_instance_is_dirty(QOF_INSTANCE(s)))
             continue;
 
-        if (i < num_preexist)
+        if (i < num_preexist && onode)
         {
             Split *so = onode->data;
 

libgnucash/engine/qofquery.cpp

@@ -486,7 +486,7 @@ compile_sort (QofQuerySort *sort, QofIdType obj)
     /* If we have valid parameters, grab the compare function,
      * If not, check if this is the default sort.
      */
-    if (sort->param_fcns)
+    if (sort->param_fcns && resObj)
     {
         /* First, check if this parameter has a sort function override.
          * if not then check if there's a global compare function for the type
@@ -535,7 +535,7 @@ static void compile_terms (QofQuery *q)
              * If not, see if this is the default sort.
              */
 
-            if (qt->param_fcns)
+            if (qt->param_fcns && resObj)
                 qt->pred_fcn = qof_query_core_get_predicate (resObj->param_type);
             else
                 qt->pred_fcn = NULL;