Fix (speculative) nullptr arithmetic

PiperOrigin-RevId: 721714171

c/dec/state.c

diff --git a/c/dec/state.c b/c/dec/state.c
index edc56c9..8b15459 100644
--- a/c/dec/state.c
+++ b/c/dec/state.c
@@ -180,7 +180,7 @@
   group->alphabet_size_limit = (uint16_t)alphabet_size_limit;
   group->num_htrees = (uint16_t)ntrees;
   group->htrees = p;
-  group->codes = (HuffmanCode*)(&p[ntrees]);
+  group->codes = p ? (HuffmanCode*)(&p[ntrees]) : NULL;
   return !!p;
 }