MDEV-18920 Prepared statements with st_convexhull hang and eat 100% cpu.

In the case of error when object shapes are half-collected we need
to set the NULL at the vertice's list.

mysql-test/r/gis-precise.result

@@ -505,6 +505,10 @@ GEOMETRYFROMTEXT('POINT(4599 60359)'),
     ) as relate_res;
 relate_res
 0
+prepare s from 'do st_convexhull(st_aswkb(multipoint(point(-11702,15179),point(-5031,27960),point(-30557,11158),point(-27804,30314))))';
+execute s;
+execute s;
+deallocate prepare s;
 DROP TABLE IF EXISTS p1;
 CREATE PROCEDURE p1(dist DOUBLE, geom TEXT)
 BEGIN

mysql-test/t/gis-precise.test

@@ -381,5 +381,11 @@ SELECT ST_RELATE(
     'F*FFFF**F'
     ) as relate_res;
 
+# MDEV-18920 Prepared statements with st_convexhull hang and eat 100% cpu.
+prepare s from 'do st_convexhull(st_aswkb(multipoint(point(-11702,15179),point(-5031,27960),point(-30557,11158),point(-27804,30314))))';
+execute s;
+execute s;
+deallocate prepare s;
+
 --source include/gis_debug.inc
 

sql/gcalc_slicescan.cc

@@ -982,6 +982,8 @@ void Gcalc_heap::reset()
 {
   if (m_n_points)
   {
+    if (m_hook)
+      *m_hook= NULL;
     free_list(m_first);
     m_n_points= 0;
   }