plugins:rtc.c: drop some invalid condition checks

There are some invalid condition checks in rtc.c, when rtc_file is
missing, it should not consider that as a error and return, otherwise,
it can lead to the case that time_set wont be called at all.

With this fix, when both RTC and file restore fail, it will fall back
to call time_set(NULL) and restore time from rtc_timestamp, this
ensures the OS has a valid time at the very first boot.

Also explicitly check strptime return value against "NULL" rather than
using "!", which is not always accurate.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>

Author: liuming50

plugins/rtc.c

@@ -173,11 +173,6 @@ static void file_restore(void *arg)
 	int rc = 1;
 	FILE *fp;
 
-	if (!rtc_file) {
-		logit(LOG_NOTICE, "System has no RTC (missing driver?), skipping restore.");
-		return;
-	}
-
 	print_desc(NULL, "Restoring system clock from backup");
 
 	fp = fopen(rtc_file, "r");
@@ -292,8 +287,7 @@ static void rtc_restore(void *arg)
 		print(2, NULL);
 
 		/* Try restoring from last save game */
-		if (rtc_file)
-			file_restore(arg);
+		file_restore(arg);
 	} else
 		print(0, NULL);
 
@@ -327,7 +321,7 @@ PLUGIN_INIT(__init)
 {
 	struct tm tm = { 0 };
 
-	if (!strptime(rtc_timestamp, RTC_FMT, &tm)) {
+	if (strptime(rtc_timestamp, RTC_FMT, &tm) == NULL) {
 		logit(LOG_WARNING, "Invalid restore date '%s', reverting to '%s'",
 		      rtc_timestamp, RTC_TIMESTAMP_BEGIN_2000);
 		rtc_timestamp = RTC_TIMESTAMP_BEGIN_2000;