wincred: avoid buffer overflow in wcsncat()

wincred: avoid buffer overflow in wcsncat()

The wincred credential helper uses a static buffer ("target") as a
unique key for storing and comparing against internal storage. It does
this by building up a string is supposed to look like:

    git:$PROTOCOL://$USERNAME@$HOST/@PATH

However, the static "target" buffer is declared as a wide string with no
more than 1,024 wide characters. The first call to wcsncat() is almost
correct (it copies no more than ARRAY_SIZE(target) wchar_t's), but does
not account for the trailing NUL, introducing an off-by-one error.

But subsequent calls to wcsncat() have an additional problem on top of
the off-by-one. They do not account for the length of the existing
wide string being built up in 'target'. So the following:

    $ perl -e '
        my $x = "x" x 1_000;
        print "protocol=$x\nhost=$x\nusername=$x\npath=$x\n"
      ' |
      C\:/Program\ Files/Git/mingw64/libexec/git-core/git-credential-wincred.exe get

will result in a segmentation fault from over-filling buffer.

This bug is as old as the wincred helper itself, dating back to
a6253da0f3 (contrib: add win32 credential-helper, 2012-07-27). Commit
8b2d219a3d (wincred: improve compatibility with windows versions,
2013-01-10) replaced the use of strncat() with wcsncat(), but retained
the buggy behavior.

Fix this by using a "target_append()" helper which accounts for both the
length of the existing string within the buffer, as well as the trailing
NUL character.

Reported-by: David Leadbeater <dgl@dgl.cx>
Helped-by: David Leadbeater <dgl@dgl.cx>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>