Set any new data allocated to 0 for clarity

This is only new data allocated, so it's a very careful procedure.

lib/darr.c

@@ -34,6 +34,7 @@ void darr_ensure_capacity(darr_t *darr, size_t requested)
     darr->available =
         MAX(darr->used + requested, darr->available * DARR_REALLOC_MULT);
     darr->data = realloc(darr->data, darr->available);
+    memset(darr->data + darr->used, 0, darr->available - darr->used);
   }
 }