Resolve some of clang's zero-as-null-pointer-constant warnings

src/hb-dsalgs.hh

diff --git a/src/hb-dsalgs.hh b/src/hb-dsalgs.hh
index e413847..9586871 100644
--- a/src/hb-dsalgs.hh
+++ b/src/hb-dsalgs.hh
@@ -49,7 +49,7 @@
     else
       return (void *) p;
   }
-  return NULL;
+  return nullptr;
 }