Replace NULL with nullptr

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

Was originally part of c67f9a4cf0d7894f7b49f70806ac323630ba178e
but was dropped when merging to aosp.

Test: m
Bug: 68236239
Change-Id: Iaba035716673d38f31ab6beb0cef431747349f72
diff --git a/cmds/dumpstate/DumpstateUtil.cpp b/cmds/dumpstate/DumpstateUtil.cpp
index 85eb464..600a500 100644
--- a/cmds/dumpstate/DumpstateUtil.cpp
+++ b/cmds/dumpstate/DumpstateUtil.cpp
@@ -56,11 +56,11 @@
     timespec ts;
     ts.tv_sec = MSEC_TO_SEC(timeout_ms);
     ts.tv_nsec = (timeout_ms % 1000) * 1000000;
-    int ret = TEMP_FAILURE_RETRY(sigtimedwait(&child_mask, NULL, &ts));
+    int ret = TEMP_FAILURE_RETRY(sigtimedwait(&child_mask, nullptr, &ts));
     int saved_errno = errno;
 
     // Set the signals back the way they were.
-    if (sigprocmask(SIG_SETMASK, &old_mask, NULL) == -1) {
+    if (sigprocmask(SIG_SETMASK, &old_mask, nullptr) == -1) {
         printf("*** sigprocmask failed: %s\n", strerror(errno));
         if (ret == 0) {
             return false;
@@ -310,7 +310,7 @@
         struct sigaction sigact;
         memset(&sigact, 0, sizeof(sigact));
         sigact.sa_handler = SIG_IGN;
-        sigaction(SIGPIPE, &sigact, NULL);
+        sigaction(SIGPIPE, &sigact, nullptr);
 
         execvp(path, (char**)args.data());
         // execvp's result will be handled after waitpid_with_timeout() below, but