hexdcoct: dump to stdout if FILE* is specified as NULL

We do a logic like that at various other places, let's do it here too,
to make this as little surprising as possible.

src/basic/hexdecoct.c

@@ -737,7 +737,10 @@ void hexdump(FILE *f, const void *p, size_t s) {
         const uint8_t *b = p;
         unsigned n = 0;
 
-        assert(s == 0 || b);
+        assert(b || s == 0);
+
+        if (!f)
+                f = stdout;
 
         while (s > 0) {
                 size_t i;