Make mojo buffer allocators more robust.

mojo/public/cpp/bindings/tests/buffer_unittest.cc

Index: mojo/public/cpp/bindings/tests/buffer_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/buffer_unittest.cc b/mojo/public/cpp/bindings/tests/buffer_unittest.cc
index 49753e1a27d576a2f1a77b4296fd6cf8f1a5172d..7ef7520f5aec054eea41cdebbb3c9e37e7c222d5 100644
--- a/mojo/public/cpp/bindings/tests/buffer_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/buffer_unittest.cc
@@ -43,6 +43,14 @@ TEST(ScratchBufferTest, Basic) {
   small = buf.Allocate(10);
   EXPECT_TRUE(IsZero(small, 10));
   EXPECT_TRUE(small >= &buf && small < (&buf + sizeof(buf)));
+
+  // And a request so large it will fail.
+  void* fail = buf.Allocate(std::numeric_limits<size_t>::max() - 1024u);
+  EXPECT_TRUE(!fail);
+
+  // And a request so large it will overflow and fail.
+  void* overflow = buf.Allocate(std::numeric_limits<size_t>::max() - 12u);
+  EXPECT_TRUE(!overflow);
 }
 
 // Tests that Buffer::current() returns the correct value.
@@ -125,6 +133,10 @@ TEST(FixedBufferTest, TooBig) {
   // Move the cursor forward.
   EXPECT_NE(reinterpret_cast<void*>(0), buf.Allocate(16));
 
+  // A lot too large.
+  EXPECT_EQ(reinterpret_cast<void*>(0),
+            buf.Allocate(std::numeric_limits<size_t>::max() - 1024u));
+
   // A lot too large, leading to possible integer overflow.
   EXPECT_EQ(reinterpret_cast<void*>(0),
             buf.Allocate(std::numeric_limits<size_t>::max() - 8u));