kernel: devrtc: fix use of uninitialized array in rtcextract

CID 156229: Uninitialized scalar variable (UNINIT)9.
uninit_use: Using uninitialized value bcdclock[0].

sys/src/kern/386/devrtc.c

@@ -118,6 +118,9 @@ rtcextract(void)
 			break;
 	}
 
+	if(i == 10000)
+		return -1;
+
 	/*
 	 *  convert from BCD
 	 */