Fixed NP dereference error reported by ecj in UploadPack.stopBuffering()

Introduced via commit 3b2508b514.

Change-Id: I2b6175c095aea2868a8c302103095accde5170e3
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java

@@ -72,7 +72,6 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
-import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.errors.CorruptObjectException;
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
@@ -1588,12 +1587,10 @@ public class UploadPack {
 		private final OutputStream rawOut;
 
 		private OutputStream out;
-		@Nullable
-		private ByteArrayOutputStream buffer;
 
 		ResponseBufferedOutputStream(OutputStream rawOut) {
 			this.rawOut = rawOut;
-			this.out = this.buffer = new ByteArrayOutputStream();
+			this.out = new ByteArrayOutputStream();
 		}
 
 		@Override
@@ -1622,9 +1619,8 @@ public class UploadPack {
 		}
 
 		void stopBuffering() throws IOException {
-			if (buffer != null) {
-				buffer.writeTo(rawOut);
-				buffer = null;
+			if (out != rawOut) {
+				((ByteArrayOutputStream) out).writeTo(rawOut);
 				out = rawOut;
 			}
 		}