Fix MSan report; add a test

src/AggregateFunctions/ReservoirSamplerDeterministic.h

@@ -169,7 +169,20 @@ public:
         DB::writeIntBinary<size_t>(total_values, buf);
 
         for (size_t i = 0; i < size; ++i)
-            DB::writePODBinary(samples[i], buf);
+        {
+            /// There was a mistake in this function.
+            /// Instead of correctly serializing the elements,
+            ///  it was writing them with uninitialized padding.
+            /// Here we ensure that padding is zero without changing the protocol.
+            /// TODO: After implementation of "versioning aggregate function state",
+            /// change the serialization format.
+
+            Element elem;
+            memset(&elem, 0, sizeof(elem));
+            elem = samples[i];
+
+            DB::writePODBinary(elem, buf);
+        }
     }
 
 private:

tests/queries/0_stateless/01779_quantile_deterministic_msan.reference

@@ -0,0 +1 @@
+11447494982455782708

tests/queries/0_stateless/01779_quantile_deterministic_msan.sql

@@ -0,0 +1 @@
+SELECT cityHash64(toString(quantileDeterministicState(number, sipHash64(number)))) FROM numbers(8193);