libopensc: guard NULL tries_left in dtrust PIN info

sc_pkcs15_get_pin_info() passes tries_left == NULL to sc_pin_cmd().
In dtrust_pin_cmd_get_info(), the CAN branch dereferenced tries_left unconditionally,
which caused a SIGSEGV during pkcs11-tool -L token enumeration.
Guard the write with a NULL check.

Fixes OpenSC#3586

Author: daloic

src/libopensc/card-dtrust.c

@@ -609,7 +609,9 @@ dtrust_pin_cmd_get_info(struct sc_card *card,
 	switch (data->pin_reference) {
 	case PACE_PIN_ID_CAN:
 		/* unlimited number of retries */
-		*tries_left = -1;
+		if (tries_left != NULL) {
+			*tries_left = -1;
+		}
 		data->pin1.max_tries = -1;
 		data->pin1.tries_left = -1;
 		r = SC_SUCCESS;