LP:823930 - Wrong result with semijoin materialization and blob fields

Two test cases from subselect_sj_mat that test materialization
with blobs produce incorrect empty result with semijoin
materialization. The two failing test cases are the ones that test
blobs with size 1024, and 1025. Here I extracted and simplified
a bit the test case marked with:

1. BLOB == 1024 (group_concat_max_len == 1024).
   For length 1025 the wrong result is the same.