Assert on _all_ failures from RAND_bytes().

Previously, we would detect errors from a missing RNG
implementation, but not failures from the RNG code itself.

Fortunately, it appears those failures do not happen in practice
when Tor is using OpenSSL's default RNG implementation.  Fixes bug
40390; bugfix on 0.2.8.1-alpha. This issue is also tracked as
TROVE-2021-004. Reported by Jann Horn at Google's Project Zero.

changes/ticket40390

+  o Major bugfixes (security, defense-in-depth):
+    - Detect a wider variety of failure conditions from the OpenSSL RNG
+      code. Previously, we would detect errors from a missing RNG
+      implementation, but not failures from the RNG code itself.
+      Fortunately, it appears those failures do not happen in practice
+      when Tor is using OpenSSL's default RNG implementation.
+      Fixes bug 40390; bugfix on 0.2.8.1-alpha. This issue is also tracked as
+      TROVE-2021-004. Reported by Jann Horn at Google's Project Zero.

src/lib/crypt_ops/crypto_rand.c

@@ -525,7 +525,7 @@ crypto_rand_unmocked(char *to, size_t n)
   /* We consider a PRNG failure non-survivable. Let's assert so that we get a
    * stack trace about where it happened.
    */
-  tor_assert(r >= 0);
+  tor_assert(r == 1);
 #endif
 }