dtrace: resolve conversion warnings from SLURP_INT

Resolve build warnings on Windows with the following pattern:
warning C4244: '=': conversion from 'int64_t' to 'int32_t',
possible loss of data

src/node_dtrace.cc

@@ -57,7 +57,7 @@ using v8::Value;
       "expected object for " #obj " to contain integer member " #member); \
   } \
   *valp = obj->Get(OneByteString(env->isolate(), #member)) \
-      ->ToInteger(env->isolate())->Value();
+      ->Int32Value();
 
 #define SLURP_OBJECT(obj, member, valp) \
   if (!(obj)->IsObject()) { \