Bugfix: Initialize temporary stratum work

Without this, work.mandatory might have been true skipping block change handling code
This caused newfound shares to be considered stale always, and bitforce devices to constantly restart work

cgminer.c

@@ -4150,6 +4150,7 @@ static void *stratum_thread(void *userdata)
 		free(s);
 		if (pool->swork.clean) {
 			struct work work;
+			memset(&work, 0, sizeof(work));
 
 			/* Generate a single work item to update the current
 			 * block database */