wolfssl: when SSL_read() returns zero, check the error

Returning zero indicates end of connection, so if there's no data read
but the connection is alive, it needs to return -1 with CURLE_AGAIN.

Closes #8431

lib/vtls/wolfssl.c

@@ -917,7 +917,7 @@ static ssize_t wolfssl_recv(struct Curl_easy *data,
 
   nread = SSL_read(backend->handle, buf, buffsize);
 
-  if(nread < 0) {
+  if(nread <= 0) {
     int err = SSL_get_error(backend->handle, nread);
 
     switch(err) {