MDEV-22243 type_test.type_test_double fails with 'NUMERIC_SCALE NULL'

1. fix useless correction of dec_arg *after it was used*
2. use DECIMAL_NOT_SPECIFIED instead of NOT_FIXED_DEC, because the latter
   is defined conditionally and has a wrong value in plugins

plugin/type_test/mysql-test/type_test/type_test_double.result

@@ -290,7 +290,7 @@ DATA_TYPE	test_double
 CHARACTER_MAXIMUM_LENGTH	NULL
 CHARACTER_OCTET_LENGTH	NULL
 NUMERIC_PRECISION	22
-NUMERIC_SCALE	31
+NUMERIC_SCALE	NULL
 DATETIME_PRECISION	NULL
 CHARACTER_SET_NAME	NULL
 COLLATION_NAME	NULL

sql/field.h

@@ -2105,7 +2105,7 @@ class Field_str :public Field {
 	    uchar null_bit_arg, utype unireg_check_arg,
 	    const LEX_CSTRING *field_name_arg,
 	    const DTCollation &collation);
-  uint decimals() const override { return is_created_from_null_item ? 0 : NOT_FIXED_DEC; }
+  uint decimals() const override { return is_created_from_null_item ? 0 : DECIMAL_NOT_SPECIFIED; }
   int  save_in_field(Field *to) override { return save_in_field_str(to); }
   bool memcpy_field_possible(const Field *from) const override
   {
@@ -2267,7 +2267,7 @@ class Field_real :public Field_num {
   Information_schema_numeric_attributes
     information_schema_numeric_attributes() const override
   {
-    return dec == NOT_FIXED_DEC ?
+    return dec == DECIMAL_NOT_SPECIFIED ?
                   Information_schema_numeric_attributes(field_length) :
                   Information_schema_numeric_attributes(field_length, dec);
   }
@@ -2836,6 +2836,8 @@ class Field_vers_trx_id :public Field_longlong {
      integers. But in all other cases we treat it as TIME_RESULT! */
 };
 
+static inline uint8 fix_dec_arg(uint8 dec_arg)
+{ return dec_arg >= FLOATING_POINT_DECIMALS ? DECIMAL_NOT_SPECIFIED : dec_arg; }
 
 class Field_float final :public Field_real {
 public:
@@ -2845,19 +2847,13 @@ class Field_float final :public Field_real {
               uint8 dec_arg,bool zero_arg,bool unsigned_arg)
     :Field_real(ptr_arg, len_arg, null_ptr_arg, null_bit_arg,
                 unireg_check_arg, field_name_arg,
-                dec_arg, zero_arg, unsigned_arg)
-    {
-      if (dec_arg >= FLOATING_POINT_DECIMALS)
-        dec_arg= NOT_FIXED_DEC;
-    }
+                fix_dec_arg(dec_arg), zero_arg, unsigned_arg)
+    { }
   Field_float(uint32 len_arg, bool maybe_null_arg,
               const LEX_CSTRING *field_name_arg, uint8 dec_arg)
     :Field_real((uchar*) 0, len_arg, maybe_null_arg ? (uchar*) "": 0, (uint) 0,
-                NONE, field_name_arg, dec_arg, 0, 0)
-    {
-      if (dec_arg >= FLOATING_POINT_DECIMALS)
-        dec_arg= NOT_FIXED_DEC;
-    }
+                NONE, field_name_arg, fix_dec_arg(dec_arg), 0, 0)
+    { }
   const Type_handler *type_handler() const override
   { return &type_handler_float; }
   enum ha_base_keytype key_type() const override { return HA_KEYTYPE_FLOAT; }
@@ -2893,28 +2889,20 @@ class Field_double :public Field_real {
 	       uint8 dec_arg,bool zero_arg,bool unsigned_arg)
     :Field_real(ptr_arg, len_arg, null_ptr_arg, null_bit_arg,
                 unireg_check_arg, field_name_arg,
-                dec_arg, zero_arg, unsigned_arg)
-    {
-      if (dec_arg >= FLOATING_POINT_DECIMALS)
-        dec_arg= NOT_FIXED_DEC;
-    }
+                fix_dec_arg(dec_arg), zero_arg, unsigned_arg)
+    { }
   Field_double(uint32 len_arg, bool maybe_null_arg,
                const LEX_CSTRING *field_name_arg, uint8 dec_arg)
     :Field_real((uchar*) 0, len_arg, maybe_null_arg ? (uchar*) "" : 0, (uint) 0,
-                NONE, field_name_arg, dec_arg, 0, 0)
-    {
-      if (dec_arg >= FLOATING_POINT_DECIMALS)
-        dec_arg= NOT_FIXED_DEC;
-    }
+                NONE, field_name_arg, fix_dec_arg(dec_arg), 0, 0)
+    { }
   Field_double(uint32 len_arg, bool maybe_null_arg,
                const LEX_CSTRING *field_name_arg,
 	       uint8 dec_arg, bool not_fixed_arg)
     :Field_real((uchar*) 0, len_arg, maybe_null_arg ? (uchar*) "" : 0, (uint) 0,
-                NONE, field_name_arg, dec_arg, 0, 0)
+                NONE, field_name_arg, fix_dec_arg(dec_arg), 0, 0)
     {
       not_fixed= not_fixed_arg;
-      if (dec_arg >= FLOATING_POINT_DECIMALS)
-        dec_arg= NOT_FIXED_DEC;
     }
   void init_for_tmp_table(Field *org_field, TABLE *new_table) override
   {