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
__cifs_sfu_make_node without checks, which passes it unchecked to
cifs_strndup_to_utf16, which in turn passes it to
cifs_local_to_utf16_bytes where '*from' is dereferenced, causing a crash.

This patch adds a check for NULL 'src' in cifs_strndup_to_utf16 and
returns NULL early to prevent dereferencing NULL pointer.

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