Correctly set buffer limits after underflow

Fixes #33

src/main/java/de/undercouch/bson4jackson/io/LittleEndianInputStream.java

@@ -266,7 +266,7 @@ public String readUTF(DataInput input, int len) throws IOException {
 					int r = Math.min(len, utf8buf.remaining());
 					input.readFully(rawUtf8Buf, utf8buf.position(), r);
 					len -= r;
-					utf8buf.limit(r);
+					utf8buf.limit(utf8buf.position() + r);
 					utf8buf.rewind();
 				} else {
 					utf8buf.flip();