Bug#12637786 was fixed with rb:692 by marko. But that fix has a remaining

bug.  It added this assert;
    ut_ad(ind_field->prefix_len);
before a section of code that assumes there is a prefix_len.  

The patch replaced code that explicitly avoided this with a check for
prefix_len.  It turns out that the purge thread can get to that assert
without a prefix_len because it does not use a row_ext_t* .
When UNIV_DEBUG is not defined, the affect of this is that the purge thread
sets the dfield->len to zero and then cannot find the entry in the index to
purge.  So secondary index entries remain unpurged.

This patch does not do the assert.  Instead, it uses
    'if (ind_field->prefix_len) {...}'
around the section of code that assumes a prefix_len.  This is the way the
patch I provided to Marko did it.

The test case is simply modified to do a sleep(10) in order to give the
purge thread a chance to run. Without the code change to row0row.c, this
modified testcase will assert if InnoDB was compiled with UNIV_DEBUG.
I tried to sleep(5), but it did not always assert.

mysql-test/suite/innodb_plugin/t/innodb-index.test

@@ -475,6 +475,9 @@ CREATE TABLE t1(a INT,
 CREATE INDEX idx1 ON t1(a,v1);
 INSERT INTO t1 VALUES(9,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r,@r);
 UPDATE t1 SET a=1000;
+DELETE FROM t1;
+# Let the purge thread clean up this file.
+-- sleep 10
 DROP TABLE t1;
 
 eval set global innodb_file_per_table=$per_table;