cert-cache: Prevent crash due to integer overflow/sign change

random() allocates values in the range [0, RAND_MAX], with RAND_MAX usually
equaling INT_MAX = 2^31-1.  Previously, values between 0 and 31 were added
directly to that offset before applying`% CACHE_SIZE` to get an index into
the cache array.  If the random value was very high, this resulted in an
integer overflow and a negative index value and, therefore, an out-of-bounds
access of the array and in turn dereferencing invalid pointers when trying
to acquire the read lock.  This most likely results in a segmentation fault.

Fixes: 764e8b2211 ("reimplemented certificate cache")
Fixes: CVE-2021-41991

src/libstrongswan/credentials/sets/cert_cache.c

@@ -151,7 +151,7 @@ static void cache(private_cert_cache_t *this,
 	for (try = 0; try < REPLACE_TRIES; try++)
 	{
 		/* replace a random relation */
-		offset = random();
+		offset = random() % CACHE_SIZE;
 		for (i = 0; i < CACHE_SIZE; i++)
 		{
 			rel = &this->relations[(i + offset) % CACHE_SIZE];