advapi32: Fix a typo (spotted by Peter Berg Larsen).
diff --git a/dlls/advapi32/crypt_arc4.c b/dlls/advapi32/crypt_arc4.c
index 2ba2b1a..9ee4b5e 100644
--- a/dlls/advapi32/crypt_arc4.c
+++ b/dlls/advapi32/crypt_arc4.c
@@ -103,7 +103,7 @@
     arc4_info a4i;
 
     arc4_init(&a4i, key->Buffer, key->Length);
-    arc4_ProcessString(&a4i, data->Buffer, key->Length);
+    arc4_ProcessString(&a4i, data->Buffer, data->Length);
 
     return STATUS_SUCCESS;
 }