unaddr in 32-bit fuzz_threads test on Xenial

fuzz_threads on Xenial fails with an unaddr:

```
Error #1: UNADDRESSABLE ACCESS beyond heap bounds: reading 0x0804ebc9-0x0804ebcc 3 byte(s) within 0x0804ebc8-0x0804ebcc
# 0 libc.so.6!? (0xf7880851 )
??:0
# 1 libc.so.6!_IO_puts (0xf786b15c )
??:0
# 2 print_buffer (0x080489dc )
??:0
# 3 thread_main (0x08048a6e )
??:0
# 4 libpthread.so.0!start_thread (0xf79c626a )
/build/glibc-LK5gWL/glibc-2.23/nptl/pthread_create.c:333
# 5 libc.so.6!clone (0xf78f141e )
??:0
Note: @0:00:02.169 in thread 30814
Note: next higher malloc: 0x0804ebf0-0x0804ebf9
Note: refers to 0 byte(s) beyond last valid byte in prior malloc
Note: prev lower malloc: 0x0804ebb8-0x0804ebc9
Note: instruction: mov (%eax) -> %ecx
Note: fuzz input for error #1 is stored in file /home/travis/build/DynamoRIO/drmemory/build_drmemory-dbg-32/logs/DrMemory-fuzz_threads.30808.000/fuzz.fuzz_threads.30808.0000.error.1.dat
```

This needs investigation. For now I'm making Travis green by adding to the wrapper list.