Make internal error check for unrecognized digest algorithm more robust

Fixes Coverity CID 479.

changes/cov479

@@ -0,0 +1,5 @@
+  o Minor bugfixes:
+    - Fix internal bug-checking logic that was supposed to catch
+      failures in digest generation so that it will fail more robustly
+      if we ask for a nonexistent algorithm. Found by Coverity Scan.
+      Bugfix on 0.2.2.1-alpha; fixes Coverity CID 479.

src/common/crypto.c

@@ -1663,6 +1663,10 @@ crypto_digest_get_digest(crypto_digest_env_t *digest,
       SHA256_Final(r, &tmpenv.d.sha2);
       break;
     default:
+      log_warn(LD_BUG, "Called with unknown algorithm %d", digest->algorithm);
+      /* If fragile_assert is not enabled, then we should at least not
+       * leak anything. */
+      memset(r, 0xff, sizeof(r));
       tor_fragile_assert();
       break;
   }