[scudo][standalone] Fix for releaseToOS prior to init

[cryptoad]

Summary

cferris@ found an issue where calling releaseToOS prior to any other
heap operation would lead to a crash, due to the allocator not being
properly initialized (it was discovered via mallopt).

The fix is to call initThreadMaybe prior to calling releaseToOS for
the Primary.

Add a test that crashes prior to fix.

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

public:
   void printStats() {
     ScopedString Str(1024);
     disable();
     getStats(&Str);
     enable();
     Str.output();
   }
 
-  void releaseToOS() { Primary.releaseToOS(); }
+  void releaseToOS() {
+    initThreadMaybe();
+    Primary.releaseToOS();
+  }
 
   // Iterate over all chunks and call a callback for all busy chunks located
   // within the provided memory range. Said callback must not use this allocator
   // or a deadlock can ensue. This fits Android's malloc_iterate() needs.
   void iterateOverChunks(uptr Base, uptr Size, iterate_callback Callback,
                          void *Arg) {
     initThreadMaybe();
     const uptr From = Base;

compiler-rt/lib/scudo/standalone/tests/combined_test.cpp

TEST(ScudoCombinedTest, DeathCombined) {
   *H ^= 0x420000U;
 
   // Invalid chunk state.
   Allocator->deallocate(P, Origin, Size);
   EXPECT_DEATH(Allocator->deallocate(P, Origin, Size), "");
   EXPECT_DEATH(Allocator->reallocate(P, Size * 2U), "");
   EXPECT_DEATH(Allocator->getUsableSize(P), "");
 }
+
+// Ensure that releaseToOS can be called prior to any other allocator
+// operation without issue.
+TEST(ScudoCombinedTest, ReleaseToOS) {
+  using AllocatorT = scudo::Allocator<DeathConfig>;
+  auto Deleter = [](AllocatorT *A) {
+    A->unmapTestOnly();
+    delete A;
+  };
+  std::unique_ptr<AllocatorT, decltype(Deleter)> Allocator(new AllocatorT,
+                                                           Deleter);
+  Allocator->reset();
+
+  Allocator->releaseToOS();
+}