tls-gnutls.c: Do not check for errno after I/O operations

Based on gnutls_record_send/recv man pages, we should use the return
value of the functions as indicator what happened in the function and do
not look into errno at all.

Checking the errno value caused infinity loop in cupsd on busy servers
if there were enough connection errors when cupsd wrote the response.

The patch is provided by Paul Zirnik from SUSE - thank you for the
patch!

Fixes #827

Author: zdohnal

cups/tls-gnutls.c

@@ -1613,7 +1613,7 @@ _httpTLSRead(http_t *http,		// I - Connection to server
 
   result = gnutls_record_recv(http->tls, buf, (size_t)len);
 
-  if (result < 0 && !errno)
+  if (result < 0)
   {
     // Convert GNU TLS error to errno value...
     switch (result)
@@ -2022,7 +2022,7 @@ _httpTLSWrite(http_t     *http,		// I - Connection to server
 
   result = gnutls_record_send(http->tls, buf, (size_t)len);
 
-  if (result < 0 && !errno)
+  if (result < 0)
   {
     // Convert GNU TLS error to errno value...
     switch (result)