Use nullptr instead of NULL in C++.

src/util.cc

@@ -737,15 +737,15 @@ char *get_exec_path(int argc, char **const argv, const char *cwd) {
 
   if (argv0[0] == '/') {
     path = static_cast<char *>(malloc(len + 1));
-    if (path == NULL) {
-        return NULL;
+    if (path == nullptr) {
+        return nullptr;
     }
     memcpy(path, argv0, len + 1);
   } else {
     auto cwdlen = strlen(cwd);
     path = static_cast<char *>(malloc(len + 1 + cwdlen + 1));
-    if (path == NULL) {
-        return NULL;
+    if (path == nullptr) {
+        return nullptr;
     }
     memcpy(path, cwd, cwdlen);
     path[cwdlen] = '/';