Fix ByteBufferBsonOutput buffer caching logic. (#1683)

Replaced getNextByteBuffer with getCurrentByteBuffer in writeOnBuffers to correct
buffer state management. getNextByteBuffer failed to update currentByteBuffer field,
causing encoding errors.

JAVA-5816

Author: vbabanin

driver-core/src/main/com/mongodb/internal/connection/ByteBufferBsonOutput.java

@@ -26,7 +26,6 @@
 import java.util.ArrayList;
 import java.util.List;
 
-import static com.mongodb.assertions.Assertions.assertFalse;
 import static com.mongodb.assertions.Assertions.assertTrue;
 import static com.mongodb.assertions.Assertions.notNull;
 import static java.lang.String.format;
@@ -176,6 +175,7 @@ private ByteBuf getCurrentByteBuffer() {
         if (currentByteBuffer == null) {
             currentByteBuffer = getByteBufferAtIndex(curBufferIndex);
         }
+
         if (currentByteBuffer.hasRemaining()) {
             return currentByteBuffer;
         }
@@ -185,11 +185,6 @@ private ByteBuf getCurrentByteBuffer() {
         return currentByteBuffer;
     }
 
-    private ByteBuf getNextByteBuffer() {
-        assertFalse(bufferList.get(curBufferIndex).hasRemaining());
-        return getByteBufferAtIndex(++curBufferIndex);
-    }
-
     private ByteBuf getByteBufferAtIndex(final int index) {
         if (bufferList.size() < index + 1) {
             ByteBuf buffer = bufferProvider.getBuffer(index >= (MAX_SHIFT - INITIAL_SHIFT)
@@ -459,7 +454,7 @@ private int writeOnBuffers(final String str,
 
             if (c < 0x80) {
                 if (remaining == 0) {
-                    curBuffer = getNextByteBuffer();
+                    curBuffer = getCurrentByteBuffer();
                     curBufferPos = 0;
                     curBufferLimit = curBuffer.limit();
                 }