Windows: do not update file timestamp with invalid value

BasiliskII/src/Windows/posix_emu.cpp

@@ -1135,6 +1135,11 @@ int my_write( int fd, const void *buffer, unsigned int count )
 
 static FILETIME get_file_time(time_t time) {
 	FILETIME ft;
+	if (time == -1) {
+		ft.dwHighDateTime = 0;
+		ft.dwLowDateTime = 0;
+		return ft;
+	}
 	unsigned long long result = 11644473600LL;
 	result += time;
 	result *= 10000000LL;