scudo: Call setCurrentTSD(nullptr) when bringing down the TSD registry in tests.

Otherwise, we will hit a use-after-free when testing multiple instances of
the same allocator on the same thread. This only recently became a problem
with D70552 which caused us to run both ScudoCombinedTest.BasicCombined and
ScudoCombinedTest.ReleaseToOS on the unit tests' main thread.

Differential Revision: https://reviews.llvm.org/D70760

compiler-rt/lib/scudo/standalone/tsd_shared.h

@@ -50,6 +50,7 @@ template <class Allocator, u32 MaxTSDCount> struct TSDRegistrySharedT {
   void unmapTestOnly() {
     unmap(reinterpret_cast<void *>(TSDs),
           sizeof(TSD<Allocator>) * NumberOfTSDs);
+    setCurrentTSD(nullptr);
   }
 
   ALWAYS_INLINE void initThreadMaybe(Allocator *Instance,