fromwire: return NULL if array empty

libwally's API requires us to pass in NULL pointers if the array size is
zero, so we update our array from wire-er to comply with this
requirement

[ Added fix to avoid tal_resize() of NULL -- RR ]

common/features.c

@@ -195,8 +195,9 @@ u8 *get_agreed_channelfeatures(const tal_t *ctx,
 		max_len = (i / 8) + 1;
 	}
 
-	/* Trim to length */
-	tal_resize(&f, max_len);
+	/* Trim to length (unless it's already NULL). */
+	if (f)
+		tal_resize(&f, max_len);
 	return f;
 }