fix integer overflows in time_t/struct tm conversion code

as found and reported by Brian Mastenbrook, the expressions
400*qc_cycles and years+100 in __secs_to_tm were both subject to
integer overflow for extreme values of the input t.

this patch by Szabolcs Nagy fixes the code by switching to larger
types, and matches the original intent I had in mind when writing this
code.

src/time/__secs_to_tm.c

@@ -10,10 +10,10 @@
 
 int __secs_to_tm(long long t, struct tm *tm)
 {
-	long long days, secs;
+	long long days, secs, years;
 	int remdays, remsecs, remyears;
 	int qc_cycles, c_cycles, q_cycles;
-	int years, months;
+	int months;
 	int wday, yday, leap;
 	static const char days_in_month[] = {31,30,31,30,31,31,30,31,30,31,31,29};
 
@@ -55,7 +55,7 @@ int __secs_to_tm(long long t, struct tm *tm)
 	yday = remdays + 31 + 28 + leap;
 	if (yday >= 365+leap) yday -= 365+leap;
 
-	years = remyears + 4*q_cycles + 100*c_cycles + 400*qc_cycles;
+	years = remyears + 4*q_cycles + 100*c_cycles + 400LL*qc_cycles;
 
 	for (months=0; days_in_month[months] <= remdays; months++)
 		remdays -= days_in_month[months];