Change NULLs to nullptrs.

src/common/misc.cpp

@@ -23,9 +23,9 @@ const char* GetLastErrorMsg()
 #ifdef _WIN32
     static __declspec(thread) char err_str[buff_size] = {};
 
-    FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM, NULL, GetLastError(),
+    FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM, nullptr, GetLastError(),
         MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
-        err_str, buff_size, NULL);
+        err_str, buff_size, nullptr);
 #else
     static __thread char err_str[buff_size] = {};