ASSERT in client.drx_buf-test: !lock->owner

http://dynamorio.org/CDash/testDetails.php?test=241352&build=21723

```
Starting drx_buf threaded test
owner
(Error occurred @3876 frags)
version 6.2.17267, custom build
-no_dynamic_options -client_lib '/data/local/tmp/build_android-debug-internal-32/suite/tests/bin/libclient.drx_buf-test.dll.so;0;' -code_api -stderr_mask 12 -stack_size 56K -max_elide_jmp 0 -max_elide_call 0 -early_inject -emulate_brk -no_inline_ignored_syscalls -native_exec_default_list '' -no_native_exec_managed_code -
0xbeae7a2c 0x00000001
0xb6cea68f 0xbd46bd77
/data/local/tmp/build_android-debug-internal-32/suite/tests/bin/libclient.drx_buf-test.dll.so=0xb6cd1000
/system/lib/libc.so=0xb6c14000
/system/lib/libdl.so=0xb6c0f000
/data/local/tmp/build_android-debug-internal-32/ext/lib32/debug/libdrx.so=0xb6cbe000
/data/local/tmp/build_android-debug-internal-32/ext/lib32/debug/libdrreg.so=0xb6cab000
/data/local/tmp/build_android-debug-internal-32/ext/lib32/debug/libdrmgr.so=0xb6c98000>
```