Correct handling of parameter variables with NULL values in PREPARE queries

mysql-test/r/ps.result

@@ -66,6 +66,7 @@ NULL
 NULL
 NULL
 NULL
+set @nullvar=1;
 set @nullvar=NULL;
 execute stmt5 using @nullvar;
 ? + a
@@ -74,4 +75,12 @@ NULL
 NULL
 NULL
 NULL
+set @nullvar2=NULL;
+execute stmt5 using @nullvar2;
+? + a
+NULL
+NULL
+NULL
+NULL
+NULL
 drop table t1;

mysql-test/t/ps.test

@@ -68,8 +68,12 @@ execute stmt5 using @a;
 
 execute stmt5 using @no_such_var;
 
+set @nullvar=1;
 set @nullvar=NULL;
 execute stmt5 using @nullvar;
 
+set @nullvar2=NULL;
+execute stmt5 using @nullvar2;
+
 drop table t1;
 

sql/sql_prepare.cc

@@ -661,7 +661,8 @@ static bool insert_params_from_vars(Prepared_statement *stmt,
     varname= var_it++;
     if ((entry= (user_var_entry*)hash_search(&stmt->thd->user_vars, 
                                              (byte*) varname->str,
-                                             varname->length)))
+                                             varname->length))
+        && entry->value)
     {
       param->item_result_type= entry->type;
       switch (entry->type)
@@ -710,7 +711,8 @@ static bool insert_params_from_vars_with_log(Prepared_statement *stmt,
     varname= var_it++;
     if ((entry= (user_var_entry*)hash_search(&stmt->thd->user_vars, 
                                              (byte*) varname->str,
-                                             varname->length)))
+                                             varname->length))
+        && entry->value)
     {
       param->item_result_type= entry->type;
       switch (entry->type)