Add another checker.

Use nullptr where appropriate.

.clang-tidy

@@ -1 +1 @@
-Checks:          '-*,clang-diagnostic-*,google-readability-casting,readability-else-after-return'
+Checks:          '-*,clang-diagnostic-*,google-readability-casting,readability-else-after-return,performance-unnecessary-copy-initialization,bugprone-use-after-move,modernize-use-nullptr'

src/override/malloc.cc

@@ -254,7 +254,7 @@ extern "C"
     if (overflow)
     {
       errno = ENOMEM;
-      return 0;
+      return nullptr;
     }
     // Include size 0 in the first sizeclass.
     sz = ((sz - 1) >> (bits::BITS - 1)) + sz;

src/pal/pal_freebsd.h

@@ -78,7 +78,7 @@ namespace snmalloc
       size_t log2align = bits::next_pow2_bits(align);
 
       void* p = mmap(
-        NULL,
+        nullptr,
         request,
         PROT_READ | PROT_WRITE,
         MAP_PRIVATE | MAP_ANONYMOUS | MAP_ALIGNED(log2align),