ATRDecodeAtr: always initialize the return values

Always set a value to availableProtocols and currentProtocol before any
return in error.

Thanks to Maksim Ivanov for the bug report
"[Pcsclite-muscle] Missing checks of ATRDecodeAtr returns"
https://lists.infradead.org/pipermail/pcsclite-muscle/2020-July/001097.html

" Hello,

The callers of the ATRDecodeAtr() function (SCardConnect() and
SCardReconnect() in winscard.c) don't check its return value, which
might potentially cause reads of uninitialized variables
|availableProtocols| and |defaultProtocol| and unexpected side
effects.

Regards,
Maksim "

src/atrhandler.c

@@ -75,15 +75,15 @@ short ATRDecodeAtr(int *availableProtocols, int *currentProtocol,
  LogXxd(PCSC_LOG_DEBUG, "ATR: ", pucAtr, dwLength);
 #endif
 
- if (dwLength < 2)
- return 0; /** @retval 0 Atr must have TS and T0 */
-
  /*
  * Zero out the bitmasks
  */
  *availableProtocols = SCARD_PROTOCOL_UNDEFINED;
  *currentProtocol = SCARD_PROTOCOL_UNDEFINED;
 
+ if (dwLength < 2)
+ return 0; /** @retval 0 Atr must have TS and T0 */
+
  /*
  * Decode the TS byte
  */