[scudo] Skip releaseToOSMaybe if there's no byte in freelist

[Chia-hungDuan]

Summary

In primary32, the unused region will have max/min region index with 0
value and it's an invalid index. Skip releaseToOSMaybe in both primary32
and primary64 even it's M_PURGE_ALL.

[cferris]

LGTM

compiler-rt/lib/scudo/standalone/primary32.h

NOINLINE uptr releaseToOSMaybe(SizeClassInfo *Sci, uptr ClassId,
     const uptr BlockSize = getSizeByClassId(ClassId);
     const uptr PageSize = getPageSizeCached();
 
     DCHECK_GE(Sci->Stats.PoppedBlocks, Sci->Stats.PushedBlocks);
     const uptr BytesInFreeList =
         Sci->AllocatedUser -
         (Sci->Stats.PoppedBlocks - Sci->Stats.PushedBlocks) * BlockSize;
 
+    if (UNLIKELY(BytesInFreeList == 0))
+      return 0;
+
     bool MaySkip = false;
 
     if (BytesInFreeList <= Sci->ReleaseInfo.BytesInFreeListAtLastCheckpoint) {
       Sci->ReleaseInfo.BytesInFreeListAtLastCheckpoint = BytesInFreeList;
       MaySkip = true;
     }
 
     // Always update `BytesInFreeListAtLastCheckpoint` with the smallest value

compiler-rt/lib/scudo/standalone/primary64.h

NOINLINE uptr releaseToOSMaybe(RegionInfo *Region, uptr ClassId,
     const uptr BlockSize = getSizeByClassId(ClassId);
     const uptr PageSize = getPageSizeCached();
 
     DCHECK_GE(Region->Stats.PoppedBlocks, Region->Stats.PushedBlocks);
     const uptr BytesInFreeList =
         Region->AllocatedUser -
         (Region->Stats.PoppedBlocks - Region->Stats.PushedBlocks) * BlockSize;
 
+    if (UNLIKELY(BytesInFreeList == 0))
+      return 0;
+
     bool MaySkip = false;
 
     // Always update `BytesInFreeListAtLastCheckpoint` with the smallest value
     // so that we won't underestimate the releasable pages. For example, the
     // following is the region usage,
     //
     //  BytesInFreeListAtLastCheckpoint   AllocatedUser
     //                v                         v