Add DBUG_ASSERT in Field::ptr_in_record

1. Subtracting table->record[0] from record is UB (non-contiguous buffers)
2. It is very popular to use move_field_offset, which changes Field::ptr,
but leaves table->record[0] unchanged. This makes a ptr_in_record result
incorrect, since it relies on table->record[0] value.
The check ensures the result is within the queried record boundaries.

sql/field.h

@@ -1151,8 +1151,9 @@ class Field: public Value_source
   virtual void reset_fields() {}
   const uchar *ptr_in_record(const uchar *record) const
   {
-    my_ptrdiff_t l_offset= (my_ptrdiff_t) (record -  table->record[0]);
-    return ptr + l_offset;
+    my_ptrdiff_t l_offset= (my_ptrdiff_t) (ptr -  table->record[0]);
+    DBUG_ASSERT(l_offset >= 0 && table->s->rec_buff_length - l_offset > 0);
+    return record + l_offset;
   }
   virtual int set_default();