[libc] fix the extension routine that adds entropy to the random pool

Worked for short entropy but actually would end up zeroing out the new
entropy word.

lib/libc/rand.c

@@ -20,7 +20,8 @@ void rand_add_entropy(const void *buf, size_t len) {
 
     uint32_t enp = 0;
     for (size_t i = 0; i < len; i++) {
-        enp ^= ((enp << 8) | (enp >> 24)) ^ ((const uint8_t *)buf)[i];
+        uint32_t c = ((const uint8_t *)buf)[i];
+        enp = ((enp << 8) | (enp >> 24)) ^ c;
     }
 
     randseed ^= enp;