fix crash in create_zip for files without modification time

MemoryDevice::lastWriteTime returns file_time_type::min(). Converting
it to the system clock overflows, and on Windows std::localtime returns
nullptr for the resulting time, which was dereferenced
(access violation in zip_files test on MSVC).

Unknown or out of range time is now written as 1980-01-01 00:00,
the earliest MS-DOS date.

src/io/devices/ZipFileDevice.cpp

@@ -58,11 +58,22 @@ namespace {
     }
 
     uint32_t to_ms_dos_timestamp(const file_time_type& fileTime) {
+        // 1980-01-01 00:00, the earliest MS-DOS date; used for unknown time
+        // (devices without timestamps return file_time_type::min())
+        constexpr uint32_t DOS_EPOCH = (0 << 9 | 1 << 5 | 1) << 16;
+        if (fileTime == file_time_type::min()) {
+            return DOS_EPOCH;
+        }
         auto timePoint = time_point_cast<system_clock::duration>(
             fileTime - file_time_type::clock::now() + system_clock::now()
         );
         std::time_t timeT = system_clock::to_time_t(timePoint);
-        std::tm tm = *std::localtime(&timeT);
+        // localtime returns nullptr for out of range time on Windows
+        const std::tm* tmPtr = std::localtime(&timeT);
+        if (tmPtr == nullptr || tmPtr->tm_year < 80) {
+            return DOS_EPOCH;
+        }
+        const std::tm& tm = *tmPtr;
         uint16_t date = (tm.tm_year - 80) << 9 | (tm.tm_mon + 1) << 5 | tm.tm_mday;
         uint16_t time = (tm.tm_hour << 11) | (tm.tm_min << 5) | (tm.tm_sec / 2);
         return (date << 16) | time;