Bugfix: If userpass is missing a password, treat it as a null password (regression fix)

cgminer.c

@@ -660,7 +660,7 @@ static char *set_userpass(const char *arg)
 		return "Failed to find : delimited user info";
 	pool->rpc_pass = strtok(NULL, ":");
 	if (!pool->rpc_pass)
-		return "Failed to find : delimited pass info";
+		pool->rpc_pass = "";
 
 	return NULL;
 }