staging: rtl8712: unterminated string leads to read overflow

staging: rtl8712: unterminated string leads to read overflow

The memdup_user() function does not necessarily return a NUL terminated
string so this can lead to a read overflow.  Switch from memdup_user()
to strndup_user() to fix this bug.

Fixes: c6dc001f2add ("staging: r8712u: Merging Realtek's latest (v2.6.6). Various fixes.")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YDYSR+1rj26NRhvb@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>