#161 Check strftime() result in soap_dateTime2s()

[Mihail Slobodyanuk]

I ran into negative date serialization issue on Windows 10 + MinGW.
It seems tm structure offer tm_sec as unsigned and if passed time_t is -1 then strftime() can't serialize date returned by gmtime().
Attached patch for stdsoap2.cpp add strftime() result check in soap_dateTime2s() and set time to "1969-12-31T23:59:59Z" on error.

[Robert van Engelen]

Thanks for sharing the patch. The 2.8.30 release includes similar changes.