Check whether the surface is found

Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>
diff --git a/encode/svctenc.c b/encode/svctenc.c
index 54529ce..cf5b3b5 100644
--- a/encode/svctenc.c
+++ b/encode/svctenc.c
@@ -1848,6 +1848,8 @@
         }
     }
 
+    assert(found != -1);
+
     return found;
 }