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/fortify.cpp b/libc/bionic/fortify.cpp
index 3bd9e68..4b94573 100644
--- a/libc/bionic/fortify.cpp
+++ b/libc/bionic/fortify.cpp
@@ -290,7 +290,7 @@
       return const_cast<char*>(p);
     }
     if (*p == '\0') {
-      return NULL;
+      return nullptr;
     }
   }
 }
@@ -387,7 +387,7 @@
 }
 
 char* __strrchr_chk(const char* p, int ch, size_t s_len) {
-  for (const char* save = NULL;; ++p, s_len--) {
+  for (const char* save = nullptr;; ++p, s_len--) {
     if (s_len == 0) {
       __fortify_fatal("strrchr: prevented read past end of buffer");
     }