platform: fix GetFreeMemory() on 64 bits freebsd

v_free_count is defined as u_int v_free_count (struct vmmeter sys/vmmeter.h:87)
but variable info defined as unsigned long, this cause error on 64-bits systems
because higher 32 bits remain uninitialized

src/platform_freebsd.cc

@@ -156,7 +156,7 @@ int Platform::GetCPUInfo(Local<Array> *cpus) {
 
 double Platform::GetFreeMemory() {
   double pagesize = static_cast<double>(sysconf(_SC_PAGESIZE));
-  unsigned long info;
+  unsigned int info = 0;
   size_t size = sizeof(info);
 
   if (sysctlbyname("vm.stats.vm.v_free_count", &info, &size, NULL, 0) < 0) {