libarchive: Fix string concatentation in Windows mktemp implementation

Port upstream LibArchive commit "compute string pointers after
concatenation" (2014-09-25) and commit "Move variables to top of
function for non-C99 compilers" (2014-11-15) to our CMake copy.
Otherwise we may compute a pointer to memory that is about to be freed
and then compute a bad size to give to CryptGenRandom.

Inspired-by: Tim Kientzle <[email protected]>

Utilities/cmlibarchive/libarchive/archive_util.c

@@ -249,6 +249,8 @@ __archive_errx(int retvalue, const char *msg)
 int
 __archive_mktemp(const char *tmpdir)
 {
+	static const wchar_t *prefix = L"libarchive_";
+	static const wchar_t *suffix = L"XXXXXXXXXX";
 	static const wchar_t num[] = {
 		L'0', L'1', L'2', L'3', L'4', L'5', L'6', L'7',
 		L'8', L'9', L'A', L'B', L'C', L'D', L'E', L'F',
@@ -323,10 +325,10 @@ __archive_mktemp(const char *tmpdir)
 	/*
 	 * Create a temporary file.
 	 */
-	archive_wstrcat(&temp_name, L"libarchive_");
-	xp = temp_name.s + archive_strlen(&temp_name);
-	archive_wstrcat(&temp_name, L"XXXXXXXXXX");
+	archive_wstrcat(&temp_name, prefix);
+	archive_wstrcat(&temp_name, suffix);
 	ep = temp_name.s + archive_strlen(&temp_name);
+	xp = ep - wcslen(suffix);
 
 	if (!CryptAcquireContext(&hProv, NULL, NULL, PROV_RSA_FULL,
 		CRYPT_VERIFYCONTEXT)) {