MDEV-12844 numerous issues in MASTER_GTID_WAIT()

sql/item_func.cc

@@ -3959,6 +3959,7 @@ longlong Item_master_gtid_wait::val_int()
 {
   DBUG_ASSERT(fixed == 1);
   longlong result= 0;
+  String *gtid_pos = args[0]->val_str(&value);
 
   if (args[0]->null_value)
   {
@@ -3970,7 +3971,6 @@ longlong Item_master_gtid_wait::val_int()
 #ifdef HAVE_REPLICATION
   THD* thd= current_thd;
   longlong timeout_us;
-  String *gtid_pos = args[0]->val_str(&value);
 
   if (arg_count==2 && !args[1]->null_value)
     timeout_us= (longlong)(1e6*args[1]->val_real());

sql/item_func.h

@@ -1669,7 +1669,7 @@ class Item_master_gtid_wait :public Item_int_func
   Item_master_gtid_wait(THD *thd, Item *a, Item *b): Item_int_func(thd, a, b) {}
   longlong val_int();
   const char *func_name() const { return "master_gtid_wait"; }
-  void fix_length_and_dec() { max_length=10+1+10+1+20+1; maybe_null=0;}
+  void fix_length_and_dec() { max_length=2; }
   bool check_vcol_func_processor(uchar *int_arg) 
   {
     return trace_unsupported_by_check_vcol_func_processor(func_name());