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;
 }
 

wire/fromwire.c

@@ -211,6 +211,9 @@ u8 *fromwire_tal_arrn(const tal_t *ctx,
 		      const u8 **cursor, size_t *max, size_t num)
 {
 	u8 *arr;
+	if (num == 0)
+		return NULL;
+
 	if (num > *max)
 		return fromwire_fail(cursor, max);