backtrace: avoid duplicate backtrace objects.

This happened in my tal_dump(), and I couldn't see how we ended up
with object having more than one "backtrace".  Adding asserts that we
never added a second backtrace didn't trigger.

Finally I wondered if we were tal_steal() backtraces, and sure enough
we do that blinding in one place: libwally wrapping.  So fix that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/utils.c

@@ -32,8 +32,16 @@ void tal_wally_end(const tal_t *parent)
 {
 	tal_t *p;
 	while ((p = tal_first(wally_tal_ctx)) != NULL) {
-		if (p != parent)
+		if (p != parent) {
+#if DEVELOPER
+			/* Don't steal backtrace from wally_tal_ctx! */
+			if (tal_name(p) && streq(tal_name(p), "backtrace")) {
+				tal_free(p);
+				continue;
+			}
+#endif /* DEVELOPER */
 			tal_steal(parent, p);
+		}
 	}
 	wally_tal_ctx = tal_free(wally_tal_ctx);
 }