clang-tidy: replace NULL/0 with nullptr

Adopt C++11 nullptr for pointer initialization and null checks,
replacing 0/NULL. Improves readability, avoids implicit integer
conversions, and aligns with clang-tidy modernize-use-nullptr.
No functional behavior changes.

Change-Id: I12c451c6c96a788f182982c6a90a377f4ed2b568
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/slp_message_handler.cpp b/slp_message_handler.cpp
index 0535f4b..aceac21 100644
--- a/slp_message_handler.cpp
+++ b/slp_message_handler.cpp
@@ -345,7 +345,7 @@
 
     if (dirPtr.get())
     {
-        while ((dent = readdir(dirPtr.get())) != NULL)
+        while ((dent = readdir(dirPtr.get())) != nullptr)
         {
             if (dent->d_type == DT_REG) // regular file
             {