Fix SerializeFileDescriptor.

The string is not null terminated, so we cannot construct it from a
buffer without also passing the size.

Test: atest rustBinderSerializationTest
Change-Id: I7b16cda3aba873b345eef6062445e5c4bd3bfa8b
diff --git a/libs/binder/rust/tests/serialization.cpp b/libs/binder/rust/tests/serialization.cpp
index ec780f2..3f59dab 100644
--- a/libs/binder/rust/tests/serialization.cpp
+++ b/libs/binder/rust/tests/serialization.cpp
@@ -381,7 +381,7 @@
     string expected = "TestingFileDescriptors";
     vector<char> buf(expected.length());
     base::ReadFully(file_descriptors[0].release(), buf.data(), buf.size());
-    ASSERT_EQ(expected, string(buf.data()));
+    ASSERT_EQ(expected, string(buf.data(), expected.length()));
 }
 
 TEST_F(SerializationTest, SerializeIBinder) {