Zarr V2 multithreaded decoding: avoid potential memory leak / race

Fixes https://issues.oss-fuzz.com/issues/489132118

Author: rouault

frmts/zarr/zarr_v2_array.cpp

@@ -692,11 +692,17 @@ bool ZarrV2Array::LoadBlockData(const uint64_t *blockIndices, bool bUseMutex,
         CPLAssert(psFilterDecompressor);
 
         CPLStringList aosOptions;
-        for (const auto &obj : oFilter.GetChildren())
+
         {
-            aosOptions.SetNameValue(obj.GetName().c_str(),
-                                    obj.ToString().c_str());
+            // Below obj.ToString() involves dynamic memory allocation
+            std::lock_guard<std::mutex> oLock(m_oMutex);
+            for (const auto &obj : oFilter.GetChildren())
+            {
+                aosOptions.SetNameValue(obj.GetName().c_str(),
+                                        obj.ToString().c_str());
+            }
         }
+
         void *out_buffer = &abyTmpRawBlockData[0];
         size_t nOutSize = abyTmpRawBlockData.size();
         if (!psFilterDecompressor->pfnFunc(