DAPSpecParser: Get rid of this warning

smocore/deviceManager/deviceAttachmentPipeSpecParser.cpp

@@ -47,9 +47,10 @@ void DeviceManager::collateAllDapSpecs(void)
 
 void DeviceManager::parseAllDapSpecs(void)
 {
-    std::unique_ptr<FILE, decltype(&fclose)> input(
+    auto file_deleter = [](FILE* f) { if (f) fclose(f); };
+    std::unique_ptr<FILE, decltype(file_deleter)> input(
         fmemopen((void*)allDapSpecs.c_str(), allDapSpecs.size(), "r"),
-        &fclose);
+        file_deleter);
 
     if (!input)
     {