cgtest: destroy the global PRNG after every cmdgen_main().

This prevents an assertion failure when random_ref() tries to create
a new PRNG instance and finds there already is one. It also exposes
bugs in which some code path forgot to initialise the PRNG when it
was going to need it, such as the one fixed in the previous commit.

cmdgen.c

@@ -1160,6 +1160,7 @@ void test(int retval, ...)
         printf("\n");
     }
     ret = cmdgen_main(argc, argv);
+    random_clear();
 
     if (ret != retval) {
 	printf("FAILED retval (exp %d got %d):", retval, ret);

putty.h

@@ -1803,6 +1803,13 @@ extern int random_active;
  * calls random_ref on startup and random_unref on shutdown. */
 void random_ref(void);
 void random_unref(void);
+/* random_clear is equivalent to calling random_unref as many times as
+ * necessary to shut down the global PRNG instance completely. It's
+ * not needed in normal applications, but the command-line PuTTYgen
+ * test finds it useful to clean up after each invocation of the
+ * logical main() no matter whether it needed random numbers or
+ * not. */
+void random_clear(void);
 /* random_setup_special is used by PuTTYgen. It makes an extra-big
  * random number generator. */
 void random_setup_special();

sshrand.c

@@ -97,16 +97,22 @@ void random_reseed(ptrlen seed)
     prng_seed_finish(global_prng);
 }
 
-void random_unref(void)
+void random_clear(void)
 {
-    assert(random_active > 0);
-    if (random_active == 1) {
+    if (global_prng) {
         random_save_seed();
         expire_timer_context(&random_timer_ctx);
         prng_free(global_prng);
         global_prng = NULL;
+        random_active = 0;
     }
-    random_active--;
+}
+
+void random_unref(void)
+{
+    assert(random_active > 0);
+    if (--random_active == 0)
+        random_clear();
 }
 
 void random_read(void *buf, size_t size)