Update BlobCacheBufferedIndexInput::readVLong to correctly handle negative long values (#115594)

Author: ChrisHegarty

docs/changelog/115594.yaml

@@ -0,0 +1,6 @@
+pr: 115594
+summary: Update `BlobCacheBufferedIndexInput::readVLong` to correctly handle negative
+  long values
+area: Search
+type: bug
+issues: []

x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/common/BlobCacheBufferedIndexInput.java

@@ -175,7 +175,7 @@ public final int readVInt() throws IOException {
 
     @Override
     public final long readVLong() throws IOException {
-        if (9 <= buffer.remaining()) {
+        if (10 <= buffer.remaining()) {
             return ByteBufferStreamInput.readVLong(buffer);
         } else {
             return super.readVLong();