MDEV-18734 ASAN heap-use-after-free upon sorting by blob column from partitioned table

ha_partition stores records in array of m_ordered_rec_buffer and uses
it for prio queue in ordered index scan. When the records are restored
from the array the blob buffers may be already freed or rewritten.

The solution is to take temporary ownership of cached blob buffers via
String::swap(). When the record is restored from m_ordered_rec_buffer
the ownership is returned to table fields.

Cleanups:

init_record_priority_queue(): removed needless !m_ordered_rec_buffer
check as there is same assertion few lines before.

dbug_print_row() for arbitrary row pointer

sql/field.cc

@@ -8318,6 +8318,7 @@ int Field_blob::store(const char *from,uint length,CHARSET_INFO *cs)
   copy_length= copier.well_formed_copy(field_charset,
                                        (char*) value.ptr(), new_length,
                                        cs, from, length);
+  value.length(copy_length);
   Field_blob::store_length(copy_length);
   bmove(ptr+packlength,(uchar*) &tmp,sizeof(char*));