LibCore: Stop buffering when a zero-size slice is read regardless of EOF

The meaning of EOF depends on the underlying socket, but an empty slice
should terminate the buffering (otherwise we'd just spin forever).

Userland/Libraries/LibCore/Stream.h

@@ -664,6 +664,9 @@ public:
 
             if (populated_slice.contains_slow('\n'))
                 return true;
+
+            if (populated_slice.is_empty())
+                break;
         }
 
         return false;