webm_info,PrintVP9Info: quiet -Wsign-compare warning

Change-Id: Ib4ecca4c7fa5911033c4b96871fa7949113ce547
diff --git a/webm_info.cc b/webm_info.cc
index c9fad2d..7f2cb6f 100644
--- a/webm_info.cc
+++ b/webm_info.cc
@@ -711,7 +711,7 @@
 
   do {
     const size_t frame_length = (count > 0) ? sizes[i] : size;
-    if (frame_length > std::numeric_limits<int>::max() ||
+    if (frame_length > static_cast<size_t>(std::numeric_limits<int>::max()) ||
         static_cast<int>(frame_length) > size) {
       fprintf(o, " invalid VP9 frame size (%u)\n",
               static_cast<uint32_t>(frame_length));