Fix a sign extension bug in stdio.

This also lets us test the EOVERFLOW behavior, which pointed out that the
fgetpos/fsetpos return on failure has always been wrong...

Bug: http://b/24807045
Change-Id: I35273eb07c8c9155af858adb27569983397580b6
diff --git a/tests/stdio_test.cpp b/tests/stdio_test.cpp
index 7f412c1..b6f6526 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -1116,14 +1116,19 @@
   FILE* fp = funopen(nullptr, read_fn, nullptr, seek_fn, nullptr);
   ASSERT_TRUE(fp != nullptr);
   fpos_t pos;
-  ASSERT_EQ(0, fgetpos(fp, &pos));
-  ASSERT_EQ(0xfedcba12LL, pos);
+#if defined(__LP64__)
+  EXPECT_EQ(0, fgetpos(fp, &pos)) << strerror(errno);
+  EXPECT_EQ(0xfedcba12LL, pos);
+#else
+  EXPECT_EQ(-1, fgetpos(fp, &pos)) << strerror(errno);
+  EXPECT_EQ(EOVERFLOW, errno);
+#endif
 
   FILE* fp64 = funopen64(nullptr, read_fn, nullptr, seek64_fn, nullptr);
   ASSERT_TRUE(fp64 != nullptr);
   fpos64_t pos64;
-  ASSERT_EQ(0, fgetpos64(fp64, &pos64));
-  ASSERT_EQ(0xfedcba12345678, pos64);
+  EXPECT_EQ(0, fgetpos64(fp64, &pos64)) << strerror(errno);
+  EXPECT_EQ(0xfedcba12345678, pos64);
 #else
   GTEST_LOG_(INFO) << "glibc uses fopencookie instead.\n";
 #endif