hash: initialize return value before calling NSS hash functions

Some hash functions in the freebl3 library don't support truncated
digests and either return immediately with no update of the output
length, or ignore the length of the output buffer and always write whole
digest.

Initialize the return value to zero to get correct result with the
former.

This is triggered only in the hash unit test. chronyd always provides a
sufficient buffer for the digest.

hash_nss.c

@@ -78,7 +78,7 @@ HSH_Hash(int id, const unsigned char *in1, unsigned int in1_len,
     const unsigned char *in2, unsigned int in2_len,
     unsigned char *out, unsigned int out_len)
 {
-  unsigned int ret;
+  unsigned int ret = 0;
 
   NSSLOWHASH_Begin(hashes[id].context);
   NSSLOWHASH_Update(hashes[id].context, in1, in1_len);