ensure that 0 length reads return an empty array not null (#3872)

Author: Wraith2

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParserStateObject.cs

@@ -2356,9 +2356,10 @@ internal TdsOperationStatus TryReadPlpBytes(ref byte[] buff, int offset, int len
                 {
                     // if there is a snapshot which it contains a stored plp buffer take it
                     // and try to use it if it is the right length
-                    buff = TryTakeSnapshotStorage() as byte[];
-                    if (buff != null)
+                    byte[] existingBuff = TryTakeSnapshotStorage() as byte[];
+                    if (existingBuff != null)
                     {
+                        buff = existingBuff;
                         totalBytesRead = _snapshot.GetPacketDataOffset();
                     }
                 }