Remember to sleep and terminate, rather than loop for ever on a zero

VSL record.  Still not quite sure how they happen, but we shouldn't hang
no matter what.

lib/libvarnishapi/vsl.c

@@ -175,6 +175,8 @@ vsl_nextlog(struct vsl *vsl, uint32_t **pp)
 
 		if (t == 0) {
 			/* Zero-initialized VSL */
+			w += SLEEP_USEC;
+			assert(usleep(SLEEP_USEC) == 0 || errno == EINTR);
 			VRMB();
 			continue;
 		}
@@ -197,6 +199,7 @@ vsl_nextlog(struct vsl *vsl, uint32_t **pp)
 				return (-1);
 			w += SLEEP_USEC;
 			assert(usleep(SLEEP_USEC) == 0 || errno == EINTR);
+			VRMB();
 			continue;
 		}
 		if (vsl->log_ptr == vsl->log_start + 1)