MDEV-5615 crash in Gcalc_function::add_operation.

The result is EMPTY for a buffer(line, -1), but we still
need one FALSE operation to be stored in the condition.
And we actually add it but forgot to alloc memory to store it.

mysql-test/r/gis-precise.result

@@ -452,3 +452,6 @@ ST_NUMPOINTS(ST_EXTERIORRING(ST_BUFFER( POLYGONFROMTEXT( 'POLYGON( ( 0.0 -3.0,
   0.0 -3.0
 ))' ),
 136
+select astext(buffer(st_linestringfromwkb(linestring(point(-1,1), point(-1,-2))),-1));
+astext(buffer(st_linestringfromwkb(linestring(point(-1,1), point(-1,-2))),-1))
+GEOMETRYCOLLECTION EMPTY

mysql-test/t/gis-precise.test

@@ -325,3 +325,6 @@ SELECT ST_NUMPOINTS(ST_EXTERIORRING(ST_BUFFER( POLYGONFROMTEXT( 'POLYGON( ( 0.0
   0.0 -3.0
 ))' ), 3 )));
 
+# MDEV-5615 crash in Gcalc_function::add_operation
+select astext(buffer(st_linestringfromwkb(linestring(point(-1,1), point(-1,-2))),-1));
+

sql/item_geofunc.cc

@@ -857,7 +857,7 @@ String *Item_func_spatial_operation::val_str(String *str_value)
   str_value->length(0);
   str_value->q_append(srid);
 
-  if (!Geometry::create_from_opresult(&buffer1, str_value, res_receiver))
+  if (Geometry::create_from_opresult(&buffer1, str_value, res_receiver))
     goto exit;
 
 exit:
@@ -1114,6 +1114,8 @@ int Item_func_buffer::Transporter::start_line()
 {
   if (buffer_op == Gcalc_function::op_difference)
   {
+    if (m_fn->reserve_op_buffer(1))
+      return 1;
     m_fn->add_operation(Gcalc_function::op_false, 0);
     skip_line= TRUE;
     return 0;
@@ -1315,7 +1317,7 @@ String *Item_func_buffer::val_str(String *str_value)
   str_value->length(0);
   str_value->q_append(srid);
 
-  if (!Geometry::create_from_opresult(&buffer, str_value, res_receiver))
+  if (Geometry::create_from_opresult(&buffer, str_value, res_receiver))
     goto mem_error;
 
   null_value= 0;

sql/spatial.cc

@@ -298,7 +298,8 @@ int Geometry::create_from_opresult(Geometry_buffer *g_buf,
 
   res->q_append((char) wkb_ndr);
   res->q_append(geom_type);
-  return obj->init_from_opresult(res, rr.result(), rr.length());
+  return obj->init_from_opresult(res, rr.result(), rr.length()) == 0 &&
+         rr.length();
 }