libwebm: Fix parser if UID high bit is set

Cherry-picking change from https://gerrit.chromium.org/gerrit/#/c/73959/

Change-Id: I8d98543f226594b28a88d5042f43c76f1224c0bb
diff --git a/libwebm/mkvparser.cpp b/libwebm/mkvparser.cpp
index f7a34c1..7501b9f 100644
--- a/libwebm/mkvparser.cpp
+++ b/libwebm/mkvparser.cpp
@@ -4133,7 +4133,7 @@
       long long val;
       status = UnserializeInt(pReader, pos, size, val);
 
-      if (val < 0)  // error
+      if (status < 0)  // error
         return status;
 
       m_uid = static_cast<unsigned long long>(val);