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

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: 764e8b2211ce ("reimplemented certificate cache")
Fixes: CVE-2021-41991