wincred: avoid memory corruption

wincred: avoid memory corruption

`wcsncpy_s()` wants to write the terminating null character so we need
to allocate one more space for it in the target memory block.

This should fix crashes when trying to read passwords.  When this
happened, the password/token wouldn't print out and Git would therefore
ask for a new password every time.

Signed-off-by: David Macek <david.macek.0@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>