Use sanity check timestamps as entropy

src/random.cpp

@@ -298,6 +298,10 @@ bool Random_SanityCheck()
     uint64_t stop = GetPerformanceCounter();
     if (stop == start) return false;
 
+    // We called GetPerformanceCounter. Use it as entropy.
+    RAND_add((const unsigned char*)&start, sizeof(start), 1);
+    RAND_add((const unsigned char*)&stop, sizeof(stop), 1);
+
     return true;
 }