[vm] Fix memory leak in Scavenger::Scavenge

```
Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x6281a99eb78d in operator new[](unsigned long) ../../../../../../llvm-llvm-project/compiler-rt/lib/asan/asan_new_delete.cpp:89:3
    #1 0x6281a9e5ba7d in dart::Scavenger::Scavenge(dart::Thread*, dart::GCType, dart::GCReason) out/ReleaseASANX64/../../runtime/vm/heap/scavenger.cc:1890:33
    #2 0x6281a9e29cfd in dart::Heap::CollectNewSpaceGarbage(dart::Thread*, dart::GCType, dart::GCReason) out/ReleaseASANX64/../../runtime/vm/heap/heap.cc:490:18
    #3 0x6281a9e2680f in dart::Heap::CollectGarbage(dart::Thread*, dart::GCType, dart::GCReason) out/ReleaseASANX64/../../runtime/vm/heap/heap.cc:564:7
    #4 0x6281a9e2680f in dart::Heap::AllocateNew(dart::Thread*, long) out/ReleaseASANX64/../../runtime/vm/heap/heap.cc:105:7
    #5 0x6281a9bfe991 in dart::Heap::Allocate(dart::Thread*, long, dart::Heap::Space) out/ReleaseASANX64/../../runtime/vm/heap/heap.h:73:16
```

Caused by https://dart-review.googlesource.com/c/sdk/+/426582.

TEST=tools/build.py --sanitizer=asan -ax64 -mrelease runtime

Change-Id: I8f863d53359fad07bcf9b30c8dd860c93b24c50f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/426900
Reviewed-by: Ivan Inozemtsev <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>
Auto-Submit: Alexander Markov <[email protected]>

Author: alexmarkov

runtime/vm/heap/scavenger.cc

@@ -1907,6 +1907,7 @@ void Scavenger::Scavenge(Thread* thread, GCType type, GCReason reason) {
     bytes_promoted += visitor->bytes_promoted();
     delete visitor;
   }
+  delete[] visitors;
 
   if (abort_) {
     ReverseScavenge(&from);