change snappyOutputStream writeHeader order to fix when outputStream… (xerial#229)

* change snappyOutputStream  writeHeader order to fix when outputStream doest't write bytes, it will also generate a snappy header after closed

Author: Neilxzn

src/main/java/org/xerial/snappy/SnappyOutputStream.java

@@ -365,22 +365,18 @@ protected void dumpOutput()
     protected void compressInput()
             throws IOException
     {
-        if (inputCursor <= 0) {
-            return; // no need to dump
-        }
-
         if (!headerWritten) {
             outputCursor = writeHeader();
             headerWritten = true;
         }
-
+        if (inputCursor <= 0) {
+            return; // no need to dump
+        }
         // Compress and dump the buffer content
         if (!hasSufficientOutputBufferFor(inputCursor)) {
             dumpOutput();
         }
-
         writeBlockPreemble();
-
         int compressedSize = Snappy.compress(inputBuffer, 0, inputCursor, outputBuffer, outputCursor + 4);
         // Write compressed data size
         writeInt(outputBuffer, outputCursor, compressedSize);