crypto_rsa.c: CID 1439488: Resource leaks
Coverity report from 18th Sept:
```
/src/lib/crypt_ops/crypto_rsa.c: 558 in crypto_pk_read_private_key_from_filename()
552 {
553 struct stat st;
554 char *buf = read_file_to_str(keyfile, 0, &st);
555 if (!buf)
556 return -1;
557 if (st.st_size > MAX_PRIVKEY_FILE_LEN)
>>> CID 1439488: Resource leaks (RESOURCE_LEAK)
>>> Variable "buf" going out of scope leaks the storage it points to.
558 return -1;
559
560 int rv = crypto_pk_read_private_key_from_string(env, buf,
561 (ssize_t)st.st_size);
562 memwipe(buf, 0, (size_t)st.st_size);
563 tor_free(buf);
```
issue