ASSERT on re-dr-start: fragment.c:5831 !pt->could_be_linking

This happened 1x in 1000 runs of an app with static DR and static drmemtrace that does start,stop,start,stop:

```
[ RUN ] DRProfilerTest.StartThenDetach



Start profiling!

Stop profiling!
Start profiling again!
could_be_linking
(Error occurred @2508 frags)
internal version, custom build
-stack_size 56K -max_elide_jmp 0 -max_elide_call 0 -no_inline_ignored_syscalls -native_exec_default_list '' -no_native_exec_managed_code -no_indcall2direct
```