sha256: fixed potentially uninitialized variable

Closes #5414

lib/sha256.c

@@ -224,6 +224,7 @@ static void SHA256_Update(SHA256_CTX *ctx,
 static void SHA256_Final(unsigned char *digest, SHA256_CTX *ctx)
 {
   unsigned long length;
+  length = 0;
 
   CryptGetHashParam(ctx->hHash, HP_HASHVAL, NULL, &length, 0);
   if(length == SHA256_DIGEST_LENGTH)