note_base uninitialised in drx and drmgr

There's a race in initialising `note_base` and `note_base_emul` respectively in `drx` and `drmgr`. Setting default value to -1 and adding an assert reveals this.

```
test 198
Start 198: code_api|client.drx-test

198: Test command: /usr/local/google/home/sharmaabhinav/dr/build/iX-note-base-race/bin64/drrun "-s" "90" "-quiet" "-debug" "-killpg" "-stderr_mask" "0xC" "-dumpcore_mask" "0" "-code_api" "-c" "/usr/local/google/home/sharmaabhinav/dr/build/iX-note-base-race/suite/tests/bin/libclient.drx-test.dll.so" "--" "/usr/local/google/home/sharmaabhinav/dr/build/iX-note-base-race/suite/tests/bin/client.drx-test"
198: Test timeout computed to be: 1500
198: ASSERT FAILURE: /usr/local/google/home/sharmaabhinav/dr/src/iX-note-base-race/ext/drx/drx.c:92: note_base > -1 (note_base not initialized)
```