Fix error handling in POSIX version of the base::File::GetLength.

sandbox/win/src/process_mitigations_test.cc

Index: sandbox/win/src/process_mitigations_test.cc
diff --git a/sandbox/win/src/process_mitigations_test.cc b/sandbox/win/src/process_mitigations_test.cc
index 7aae5964bec3872ed1996b89398933de01eb1bae..53b0442c1c54d0cc0a0fad6d7672448f57bea114 100644
--- a/sandbox/win/src/process_mitigations_test.cc
+++ b/sandbox/win/src/process_mitigations_test.cc
@@ -1536,6 +1536,8 @@ SBOX_TESTS_COMMAND int CheckWin10FontLoad(int argc, wchar_t** argv) {
 
   std::vector<char> font_data;
   int64_t len = file.GetLength();
+  if (len < 0)
+    return SBOX_TEST_NOT_FOUND;
   font_data.resize(len);
 
   int read = file.Read(0, &font_data[0], len);