[vm] Unpoison memory before zapping.

Zapping is applied to the whole segment, not just what had been allocated, and the unallocated portion is poisoned.

TEST=debug asan
Change-Id: I2e0d42f97dda4d38fc2579b54d8e171ee88098fe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/309781
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Brian Quinlan <bquinlan@google.com>

runtime/vm/zone.cc

diff --git a/runtime/vm/zone.cc b/runtime/vm/zone.cc
index 47a42ab..a6d62c4 100644
--- a/runtime/vm/zone.cc
+++ b/runtime/vm/zone.cc
@@ -101,6 +101,7 @@
 
 #ifdef DEBUG
   // Zap the entire allocated segment (including the header).
+  ASAN_UNPOISON(reinterpret_cast<void*>(result), size);
   memset(reinterpret_cast<void*>(result), kZapUninitializedByte, size);
 #endif
   ASAN_POISON(reinterpret_cast<void*>(result), size);
@@ -125,6 +126,7 @@
     VirtualMemory* memory = current->memory();
 #ifdef DEBUG
     // Zap the entire current segment (including the header).
+    ASAN_UNPOISON(reinterpret_cast<void*>(current), current->size());
     memset(reinterpret_cast<void*>(current), kZapDeletedByte, current->size());
 #endif
     ASAN_POISON(reinterpret_cast<void*>(current), size);
@@ -179,6 +181,7 @@
   segments_ = nullptr;
 
 #ifdef DEBUG
+  ASAN_UNPOISON(&buffer_, kInitialChunkSize);
   memset(&buffer_, kZapDeletedByte, kInitialChunkSize);
 #endif
   ASAN_POISON(&buffer_, kInitialChunkSize);