Fix for bug #12281 (Geometry: crash in trigger)

Field_geom lack size_of method

sql/field.h

@@ -1204,6 +1204,7 @@ class Field_geom :public Field_blob {
   int  store(longlong nr);
   int  store_decimal(const my_decimal *);
   void get_key_image(char *buff,uint length,imagetype type);
+  uint size_of() const { return sizeof(*this); }
 };
 #endif /*HAVE_SPATIAL*/