VM: Don't do free("") in shutdown when we can't find the cpu model field in /proc/cpuinfo.

runtime/vm/cpuinfo.h

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_CPUINFO_H_
 #define VM_CPUINFO_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 
@@ skipping 43 matching lines @@
   // Returns the field. Caller is responsible for freeing the result.
   static const char* ExtractField(CpuInfoIndices idx);
   static const char* ExtractFieldByString(const char* field);
 
   // Returns the field describing the CPU model. Caller is responsible for
   // freeing the result.
   static const char* GetCpuModel() {
     if (HasField(FieldName(kCpuInfoHardware))) {
       return ExtractField(kCpuInfoHardware);
     } else {
-      return "";
+      return strdup("Unknown");
     }
   }
 
  private:
   // The method to use to acquire info about the CPU.
   static CpuInfoMethod method_;
 
   // Cpuinfo field names.
   static const char* fields_[kCpuInfoMax];
 };
 
 }  // namespace dart
 
 #endif  // VM_CPUINFO_H_