Fixed the problem of mdev-6198: statistics.test fails in valgrind.

The problem appeared when a loose scan used a key prefix whose
last component called uint3korr in the implementation of the key_cmp
virtual function.

sql/opt_range.cc

@@ -13756,14 +13756,20 @@ int QUICK_GROUP_MIN_MAX_SELECT::init()
   if (group_prefix) /* Already initialized. */
     return 0;
   
-  if (!(last_prefix= (uchar*) alloc_root(&alloc, group_prefix_len)))
+  /*
+    We allocate one byte more to serve the case when the last field in
+    the buffer is compared using uint3korr (e.g. a Field_newdate field)
+  */
+  if (!(last_prefix= (uchar*) alloc_root(&alloc, group_prefix_len+1)))
       return 1;
   /*
     We may use group_prefix to store keys with all select fields, so allocate
     enough space for it.
+    We allocate one byte more to serve the case when the last field in
+    the buffer is compared using uint3korr (e.g. a Field_newdate field)
   */
   if (!(group_prefix= (uchar*) alloc_root(&alloc,
-                                         real_prefix_len + min_max_arg_len)))
+                                          real_prefix_len+min_max_arg_len+1)))
     return 1;
 
   if (key_infix_len > 0)