CLEANUP: init: address a coverity warning about possible multiply overflow

In issue #1585 Coverity suspects a risk of multiply overflow when
calculating the SSL cache size, though in practice the cache is
limited to 2^32 anyway thus it cannot really happen. Nevertheless,
casting the operation should be sufficient to avoid marking it as a
false positive.

src/haproxy.c

@@ -2302,7 +2302,7 @@ static void init(int argc, char **argv)
 		int64_t mem = global.rlimit_memmax * 1048576ULL;
 		int retried = 0;
 
-		mem -= global.tune.sslcachesize * 200; // about 200 bytes per SSL cache entry
+		mem -= global.tune.sslcachesize * 200ULL; // about 200 bytes per SSL cache entry
 		mem -= global.maxzlibmem;
 		mem = mem * MEM_USABLE_RATIO;