bitcoin: avoid adding to NULL

Detected by UBSan:

$ UBSAN_OPTIONS=print_stacktrace=1 ./wallet/test/run-psbt_fixup

bitcoin/psbt.c:733:2: runtime error: applying zero offset to null pointer
    #0 0x53c829 in psbt_from_bytes lightning/bitcoin/psbt.c:733:2
    #1 0x5adcb0 in main lightning/wallet/test/run-psbt_fixup.c:174:10

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior bitcoin/psbt.c:733:2

common/utils.h

@@ -123,10 +123,10 @@ void tal_wally_end(const tal_t *parent);
 /* ... or this if you want to reparent onto something which is
  * allocated by libwally here.  Fixes up this from_wally obj to have a
  * proper tal_name, too! */
-#define tal_wally_end_onto(parent, from_wally, type)			\
-	tal_wally_end_onto_((parent),					\
-			    (from_wally) + 0*sizeof((from_wally) == (type *)0), \
-			    stringify(type))
+#define tal_wally_end_onto(parent, from_wally, type)                           \
+	tal_wally_end_onto_(                                                   \
+	    (parent), (from_wally),                                            \
+	    &stringify(type)[0 * sizeof((from_wally) == (type *)0)])
 void tal_wally_end_onto_(const tal_t *parent,
 			 tal_t *from_wally,
 			 const char *from_wally_name);