LibC: Automatically append null terminator in vswprintf

Author: https://github.com/safarp
Commit: b0b8d14a2c
Pull-request: https://github.com/SerenityOS/serenity/pull/13147
Reviewed-by: https://github.com/trflynn89

Userland/Libraries/LibC/wstdio.cpp

@@ -207,6 +207,10 @@ int vswprintf(wchar_t* __restrict wcs, size_t max_length, wchar_t const* __restr
         ++length_so_far;
     },
         wcs, fmt, args);
+    if (length_so_far < max_length)
+        wcs[length_so_far] = L'\0';
+    else
+        wcs[max_length - 1] = L'\0';
     return static_cast<int>(length_so_far);
 }