CID 1444908: MISSING_LOCK / CID 1444769: TAINTED_SCALAR

Got two new coverity issues:

*** CID 1444908:  Concurrent data access violations  (MISSING_LOCK)
/src/test/rng_test_helpers.c: 190 in testing_enable_prefilled_rng()
184     {
185       tor_assert(buflen > 0);
186       rng_mutex = tor_mutex_new();
187     
188       prefilled_rng_buffer = tor_memdup(buffer, buflen);
189       prefilled_rng_buflen = buflen;
>>>     CID 1444908:  Concurrent data access violations  (MISSING_LOCK)
>>>     Accessing "prefilled_rng_idx" without holding lock "tor_mutex_t.mutex". Elsewhere, "prefilled_rng_idx" is accessed with
>>> "tor_mutex_t.mutex" held 3 out of 4 times (1 of these accesses strongly imply that it is necessary).
190       prefilled_rng_idx = 0;
191     
192       MOCK(crypto_rand, crypto_rand_prefilled);
193       MOCK(crypto_strongest_rand_, mock_crypto_strongest_rand);
194     }
195     

** CID 1444769:  Insecure data handling  (TAINTED_SCALAR)

________________________________________________________________________________________________________
*** CID 1444769:  Insecure data handling  (TAINTED_SCALAR)
/src/feature/nodelist/microdesc.c: 540 in microdesc_cache_reload()
534       }
535     
536       journal_content = read_file_to_str(cache->journal_fname,
537                                          RFTS_IGNORE_MISSING, &st);
538       if (journal_content) {
539         cache->journal_len = (size_t) st.st_size;
>>>     CID 1444769:  Insecure data handling  (TAINTED_SCALAR)
>>>     Passing tainted variable "journal_content" to a tainted sink.
540         warn_if_nul_found(journal_content, cache->journal_len, 0,
541                           "reading microdesc journal");
542         added = microdescs_add_to_cache(cache, journal_content,
543                                         journal_content+st.st_size,
544                                         SAVED_IN_JOURNAL, 0, -1, NULL);
545         if (added) {
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