Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I5b4123bc6709641315120a191e36cc57541349b2
diff --git a/libc/bionic/__cmsg_nxthdr.cpp b/libc/bionic/__cmsg_nxthdr.cpp
index f962788..354e17e 100644
--- a/libc/bionic/__cmsg_nxthdr.cpp
+++ b/libc/bionic/__cmsg_nxthdr.cpp
@@ -33,7 +33,7 @@
   ptr = reinterpret_cast<cmsghdr*>(reinterpret_cast<char*>(cmsg) + CMSG_ALIGN(cmsg->cmsg_len));
   size_t len = reinterpret_cast<char*>(ptr+1) - reinterpret_cast<char*>(msg->msg_control);
   if (len > msg->msg_controllen) {
-    return NULL;
+    return nullptr;
   }
   return ptr;
 }