Fix for bug #26012: missed Field_double::size_of()

As we have Field_double::not_fixed we must have Field_double::size_of().


sql/field.h:
  Fix for bug #26012: missed Field_double::size_of()
    - Field_double::size_of() added.

sql/field.h

@@ -645,6 +645,7 @@ public:
   uint32 pack_length() const { return sizeof(double); }
   void sql_type(String &str) const;
   uint32 max_length() { return 53; }
+  uint size_of() const { return sizeof(*this); }
 };