Segfault with --debug=on and DB tests

Splitting from bug #1043629.

With --debug=on, --report-interval alone (with --tx-rate) is crashing.

raghu archie:/sysbench/sysbench/tests/db [300]% sysbench --debug=on --test=./oltp.lua --db-driver=mysql --mysql-engine-trx=yes --mysql-table-engine=innodb --mysql-user=root --mysql-password=test --oltp-table-size=5000 --num-threads=8 --report-interval=1 run sysbench 0.5: multi-threaded system evaluation benchmark

Running the test with following options:
Number of threads: 8
Report intermediate results every 1 second(s)
Debug mode enabled.

Random number generator seed is 0 and will be ignored

DEBUG: Reporting thread started
DEBUG: Runner thread started (0)!
DEBUG: Runner thread started (1)!
DEBUG: Runner thread started (3)!
DEBUG: Runner thread started (2)!
DEBUG: Runner thread started (4)!
DEBUG: Runner thread started (6)!
Threads started!

DEBUG: Runner thread started (7)!
DEBUG: Runner thread started (5)!
zsh: segmentation fault sysbench --debug=on --test=./oltp.lua --db-driver=mysql --mysql-engine-trx=ye

But without --debug it works fine.

Backtrace: http://sprunge.us/aPGa