ASSERT interp.c:5447 t->bbs != NULL in drx_buf-test

This just happened. Non-deterministic: never seen it before.

https://travis-ci.org/DynamoRIO/dynamorio/jobs/282481310
```
172/275 Test #169: code_api|client.drx_buf-test .................................***Failed Required regular expression not found.Regex=[^Starting drx_buf threaded test
Ending drx_buf threaded test
Starting drx_buf signal test
drx_buf signal test PASS
Ending drx_buf signal test
$
] 1.04 sec
Starting drx_buf threaded test
bbs != NULL
(Error occurred @2071 frags)
version 6.2.17441, custom build
-no_dynamic_options -client_lib '/home/travis/build/DynamoRIO/dynamorio/build_debug-internal-64/suite/tests/bin/libclient.drx_buf-test.dll.so;0;' -code_api -stderr_mask 12 -stack_size 56K -signal_stack_size 32K -max_elide_jmp 0 -max_elide_call 0 -early_inject -emulate_brk -no_inline_ignored_syscalls -native_exec_default_
0x00000000542409a0 0x00007f7074cc6d19
0x0000000054240be0 0x00007f7074e44b37
0x0000000054240de0 0x00007f7074dda555
0x0000000054241430 0x00007f7074ddbbbf
0x0000000054241470 0x00007f7074db55b8
0x00000000542414e0 0x00007f7074ea59bb
0x00000000542417c0 0x00007f7074eaa512
0x0000000054241a00 0x00007f7074ead1f3
0x0000000054241ab0 0x00007f7074e6fac0
0x00007f7073d70f10 0x00007f7074146184
/home/travis/build/DynamoRIO/dynamorio/build_debug-internal-64/suite/tests/bin/libclient.drx_buf-test.dll.so=0x0000000072000000
/lib/x86_64-linux-gnu/libc.so.6=0x00007f70745ba000
/lib64/ld-linux-x86-64.so.2=0x00007f7074394000
/home/travis/build/DynamoRIO/dynamorio/build_debug-internal-64/ext/lib64/debug/libdrx.so=0x0000000077000000
/home/travis/build/DynamoRIO/dynamorio/build_debug-internal-64/ext/lib64/debug/libdrreg.so=0x0000000078000000
/home>
```

@toshipiazza, have you ever seen this?