Add SandboxBPFPolicy::InvalidSyscall() to simplify writing policies

sandbox/linux/seccomp-bpf/verifier.cc

Index: sandbox/linux/seccomp-bpf/verifier.cc
diff --git a/sandbox/linux/seccomp-bpf/verifier.cc b/sandbox/linux/seccomp-bpf/verifier.cc
index 1292504decc012e9cfbc6c3e6eca39cc440b8cf6..07b13d6f61665cee15dcda1bcd80064bdfc69dcd 100644
--- a/sandbox/linux/seccomp-bpf/verifier.cc
+++ b/sandbox/linux/seccomp-bpf/verifier.cc
@@ -387,7 +387,9 @@ bool Verifier::VerifyBPF(SandboxBPF* sandbox,
     }
 #endif
 #endif
-    ErrorCode code = policy.EvaluateSyscall(sandbox, sysnum);
+    ErrorCode code = iter.IsValid(sysnum)
+                         ? policy.EvaluateSyscall(sandbox, sysnum)
+                         : policy.InvalidSyscall(sandbox);
     if (!VerifyErrorCode(sandbox, program, &data, code, code, err)) {
       return false;
     }