Comment 4 for bug 1213120

Crash is guaranteed no debug version, but might only happen sometimes on normal build

Stack dump when it does:

#2 <signal handler called>
#3 intrusive_ptr_release (ptr=<optimized out>) at /home/andrew/develop/maria/repo/oqgraph-dev/storage/oqgraph/oqgraph_thunk.h:132
#4 ~intrusive_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/boost/smart_ptr/intrusive_ptr.hpp:101
#5 operator= (rhs=<optimized out>, this=<optimized out>) at /usr/include/boost/smart_ptr/intrusive_ptr.hpp:133
#6 operator= (this=<optimized out>) at /home/andrew/develop/maria/repo/oqgraph-dev/storage/oqgraph/oqgraph_thunk.h:58
#7 operator= (this=<optimized out>) at /home/andrew/develop/maria/repo/oqgraph-dev/storage/oqgraph/oqgraph_thunk.h:68
#8 operator= (this=<optimized out>) at /home/andrew/develop/maria/repo/oqgraph-dev/storage/oqgraph/graphcore.cc:136
#9 open_query::edges_cursor::current (this=0x1caa7a0, ref=...) at /home/andrew/develop/maria/repo/oqgraph-dev/storage/oqgraph/graphcore.cc:316
#10 0x00007fe58275ae50 in open_query::oqgraph::row_ref (this=<optimized out>, ref_ptr=0x1cb14e0) at /home/andrew/develop/maria/repo/oqgraph-dev/storage/oqgraph/graphcore.cc:948
#11 0x00007fe582758fcc in ha_oqgraph::index_read (this=0x1cad470, buf=0x1cae5f0 "\377", key=0x1cb4f68 "", key_len=35, find_flag=HA_READ_KEY_EXACT) at /home/andrew/develop/maria/repo/oqgraph-dev/storage/oqgraph/ha_oqgraph.cc:748
#12 0x00000000006a0815 in handler::ha_index_read_map (this=0x1cad470, buf=0x1cae5f0 "\377", key=0x1cb4f68 "", keypart_map=1, find_flag=HA_READ_KEY_EXACT) at /home/andrew/develop/maria/repo/oqgraph-dev/sql/handler.cc:2447