tx: add missing check for NULL written param

What changed, and why it matters

This commit fixes a simple but real bug in a transaction helper function. Before the fix, if a caller accidentally passed a null pointer as the output parameter, the library would crash by writing to address zero. The fix adds a standard null check and returns an error instead. It is a defensive hardening change rather than an active remote exploit, but it removes a reliable local crash path.

Security signals we found

Evidence from the diff

Changed components

diff --git a/src/transaction.c b/src/transaction.c
index ad3c422..17f7277 100644
--- a/src/transaction.c
+++ b/src/transaction.c
@@ -1857,6 +1857,8 @@ int wally_tx_get_weight(const struct wally_tx *tx, size_t *written)
 
 int wally_tx_vsize_from_weight(size_t weight, size_t *written)
 {
+    if (!written)
+        return WALLY_EINVAL;
     *written = (weight + 3) / 4; /* ceil(weight/4) */
     return WALLY_OK;
 }