fix: temp file existence checking is wrong getObject method (#387)

Previously in getObject method, temp file existence check is wrongly
with actual file name. This causes exception when the destination file
exists.
This patch fixes the issue.

src/main/java/io/minio/MinioClient.java

@@ -1180,7 +1180,7 @@ public void getObject(String bucketName, String objectName, String fileName)
 
     String tempFileName = fileName + "." + etag + ".part.minio";
     Path tempFilePath = Paths.get(tempFileName);
-    boolean tempFileExists = Files.exists(filePath);
+    boolean tempFileExists = Files.exists(tempFilePath);
 
     if (tempFileExists && !Files.isRegularFile(tempFilePath)) {
       throw new IOException(tempFileName + ": not a regular file");