Add SandboxBPFPolicy::InvalidSyscall() to simplify writing policies

sandbox/linux/seccomp-bpf/sandbox_bpf_compatibility_policy.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SANDBOX_LINUX_SECCOMP_BPF_SANDBOX_BPF_COMPATIBILITY_POLICY_H_
 #define SANDBOX_LINUX_SECCOMP_BPF_SANDBOX_BPF_COMPATIBILITY_POLICY_H_
 
 #include "base/basictypes.h"
 #include "base/logging.h"
 #include "base/macros.h"
@@ skipping 10 matching lines @@
   typedef ErrorCode (*SyscallEvaluator)(SandboxBPF* sandbox_compiler,
                                         int system_call_number,
                                         AuxType* aux);
   CompatibilityPolicy(SyscallEvaluator syscall_evaluator, AuxType* aux)
       : syscall_evaluator_(syscall_evaluator), aux_(aux) {}
 
   virtual ~CompatibilityPolicy() {}
 
   virtual ErrorCode EvaluateSyscall(SandboxBPF* sandbox_compiler,
                                     int system_call_number) const OVERRIDE {
+    DCHECK(SandboxBPF::IsValidSyscallNumber(system_call_number));
     return syscall_evaluator_(sandbox_compiler, system_call_number, aux_);
   }
 
  private:
   SyscallEvaluator syscall_evaluator_;
   AuxType* aux_;
   DISALLOW_COPY_AND_ASSIGN(CompatibilityPolicy);
 };
 
 }  // namespace sandbox
 
 #endif  // SANDBOX_LINUX_SECCOMP_BPF_SANDBOX_BPF_COMPATIBILITY_POLICY_H_