Unchecked wc_InitRsaKey Return Value

Check the return value of `wc_InitRsaKey()`. It will initialize the
structure provided the pointer is non-null. Since the key is on the
stack, the later call to `wc_FreeRsaKey()` will succeed as well.
Modified the check for the encoded signature size inside the block where
it is set; that check also updates the return value.

Affected function: SignHashRsa.
Issue: F-212

Author: ejohnstown

src/agent.c

@@ -681,24 +681,25 @@ static int SignHashRsa(WOLFSSH_AGENT_KEY_RSA* rawKey, enum wc_HashType hashType,
 {
     RsaKey key;
     byte encSig[MAX_ENCODED_SIG_SZ];
-    int encSigSz;
-    int ret = 0;
-
-    wc_InitRsaKey(&key, heap);
-    mp_read_unsigned_bin(&key.n, rawKey->n, rawKey->nSz);
-    mp_read_unsigned_bin(&key.e, rawKey->e, rawKey->eSz);
-    mp_read_unsigned_bin(&key.d, rawKey->d, rawKey->dSz);
-    mp_read_unsigned_bin(&key.p, rawKey->p, rawKey->pSz);
-    mp_read_unsigned_bin(&key.q, rawKey->q, rawKey->qSz);
-    mp_read_unsigned_bin(&key.u, rawKey->iqmp, rawKey->iqmpSz);
-
-    encSigSz = wc_EncodeSignature(encSig, digest, digestSz,
-            wc_HashGetOID(hashType));
-    if (encSigSz <= 0) {
-        WLOG(WS_LOG_DEBUG, "Bad Encode Sig");
-        ret = WS_CRYPTO_FAILED;
+    int encSigSz, ret;
+
+    ret = wc_InitRsaKey(&key, heap);
+    if (ret == 0) {
+        mp_read_unsigned_bin(&key.n, rawKey->n, rawKey->nSz);
+        mp_read_unsigned_bin(&key.e, rawKey->e, rawKey->eSz);
+        mp_read_unsigned_bin(&key.d, rawKey->d, rawKey->dSz);
+        mp_read_unsigned_bin(&key.p, rawKey->p, rawKey->pSz);
+        mp_read_unsigned_bin(&key.q, rawKey->q, rawKey->qSz);
+        mp_read_unsigned_bin(&key.u, rawKey->iqmp, rawKey->iqmpSz);
+
+        encSigSz = wc_EncodeSignature(encSig, digest, digestSz,
+                wc_HashGetOID(hashType));
+        if (encSigSz <= 0) {
+            WLOG(WS_LOG_DEBUG, "Bad Encode Sig");
+            ret = WS_CRYPTO_FAILED;
+        }
     }
-    else {
+    if (ret == 0) {
         WLOG(WS_LOG_INFO, "Signing hash with RSA.");
         *sigSz = wc_RsaSSL_Sign(encSig, encSigSz, sig, *sigSz, &key, rng);
         if (*sigSz <= 0) {