MDEV-4285 Server crashes in ptr_compare on NOW and CAST in ORDER BY

skip qsort if the sort key has zero length

mysql-test/r/order_by_zerolength-4285.result

+create table t1 (pk int primary key);
+insert into t1 values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10);
+select * from t1 order by now(), cast(pk as char(0));
+pk
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+Warnings:
+Warning	1292	Truncated incorrect CHAR(0) value: '1'
+Warning	1292	Truncated incorrect CHAR(0) value: '2'
+Warning	1292	Truncated incorrect CHAR(0) value: '3'
+Warning	1292	Truncated incorrect CHAR(0) value: '4'
+Warning	1292	Truncated incorrect CHAR(0) value: '5'
+Warning	1292	Truncated incorrect CHAR(0) value: '6'
+Warning	1292	Truncated incorrect CHAR(0) value: '7'
+Warning	1292	Truncated incorrect CHAR(0) value: '8'
+Warning	1292	Truncated incorrect CHAR(0) value: '9'
+Warning	1292	Truncated incorrect CHAR(0) value: '10'
+drop table t1;

mysql-test/t/order_by_zerolength-4285.test

+#
+# MDEV-4285 Server crashes in ptr_compare on NOW and CAST in ORDER BY
+#
+create table t1 (pk int primary key);
+insert into t1 values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10);
+select * from t1 order by now(), cast(pk as char(0));
+drop table t1;
+

sql/filesort_utils.cc

@@ -125,7 +125,8 @@ void Filesort_buffer::free_sort_buffer()
 
 void Filesort_buffer::sort_buffer(const Sort_param *param, uint count)
 {
-  if (count <= 1)
+  size_t size= param->sort_length;
+  if (count <= 1 || size == 0)
     return;
   uchar **keys= get_sort_keys();
   uchar **buffer= NULL;
@@ -138,6 +139,5 @@ void Filesort_buffer::sort_buffer(const Sort_param *param, uint count)
     return;
   }
   
-  size_t size= param->sort_length;
   my_qsort2(keys, count, sizeof(uchar*), get_ptr_compare(size), &size);
 }