test: limit the valid image size on OS/2

Without this, test_libvpx crashes at VP9FrameSizeTestsLarge.ValidSizes
on OS/2.

Change-Id: If253fb95a04216babed56458f0fab8c70523fc7e

test/frame_size_tests.cc

@@ -74,7 +74,7 @@ TEST_F(VP9FrameSizeTestsLarge, ValidSizes) {
   // size or almost 1 gig of memory.
   // In total the allocations will exceed 2GiB which may cause a failure with
   // mingw + wine, use a smaller size in that case.
-#if defined(_WIN32) && !defined(_WIN64)
+#if defined(_WIN32) && !defined(_WIN64) || defined(__OS2__)
   video.SetSize(4096, 3072);
 #else
   video.SetSize(4096, 4096);