CVE-2025-39838

In the Linux kernel, the following vulnerability has been resolved:

cifs: prevent NULL pointer dereference in UTF16 conversion

There can be a NULL pointer dereference bug here. NULL is passed to _cifssfumakenode without checks, which passes it unchecked to cifsstrnduptoutf16, which in turn passes it to cifslocaltoutf16_bytes where '*from' is dereferenced, causing a crash.

This patch adds a check for NULL 'src' in cifsstrndupto_utf16 and returns NULL early to prevent dereferencing NULL pointer.

Found by Linux Verification Center (linuxtesting.org) with SVACE