memdump: fix overzealous assertions from b857b2e843

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

lightningd/memdump.c

@@ -73,8 +73,10 @@ static int json_add_syminfo(void *data, uintptr_t pc UNUSED,
 	struct json_result *response = data;
 	char *str;
 
-	assert(filename != NULL);
+	/* This can happen in backtraces. */
-	assert(function != NULL);
+	if (!filename || !function)
+		return 0;
+
 	str = tal_fmt(response, "%s:%u (%s)", filename, lineno, function);
 	json_add_string(response, NULL, str);
 	tal_free(str);