fix: potential crash with multi-sharded pfmerge (#5008)

Fixes #5004

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

Author: romange

src/server/hll_family.cc

@@ -255,14 +255,14 @@ OpResult<int> PFMergeInternal(CmdArgList args, Transaction* tx, SinkReplyBuilder
     if (result.ok()) {
       hlls[sid] = std::move(result.value());
     } else {
-      success = false;
+      success.store(false, memory_order_relaxed);
     }
-    return result.status();
+    return OpStatus::OK;
   };
 
   tx->Execute(std::move(cb), false);
 
-  if (!success) {
+  if (!success.load(memory_order_relaxed)) {
     tx->Conclude();
     return OpStatus::INVALID_VALUE;
   }

src/server/hll_family_test.cc

@@ -194,11 +194,12 @@ TEST_F(HllFamilyTest, MergeOverlapping) {
 }
 
 TEST_F(HllFamilyTest, MergeInvalid) {
-  GTEST_SKIP() << "TBD: MergeInvalid test fails with multi-shard runs, see #5004";
+  Run({"exists", "key1", "key4"});
+  ASSERT_EQ(GetDebugInfo().shards_count, 2);  // ensure 2 shards
 
   EXPECT_EQ(CheckedInt({"pfadd", "key1", "1", "2", "3"}), 1);
-  EXPECT_EQ(Run({"set", "key2", "..."}), "OK");
-  EXPECT_THAT(Run({"pfmerge", "key1", "key2"}), ErrArg(HllFamily::kInvalidHllErr));
+  EXPECT_EQ(Run({"set", "key4", "..."}), "OK");
+  EXPECT_THAT(Run({"pfmerge", "key1", "key4"}), ErrArg(HllFamily::kInvalidHllErr));
   EXPECT_EQ(CheckedInt({"pfcount", "key1"}), 3);
 }