Crash in TGaxis::PaintAxis() in certain cases when using time format

When using time format in axis, TGaxis::PaintAxis() may in some cases call strftime() with invalid parameter causing a crash. Below is a test case that crashes root.exe in Windows using ROOT 5.34/18:

TGraph graph;

graph.SetPoint(0, -1, 0);

graph.GetXaxis()->SetTimeFormat("%y-%m-%d %H:%M%S%F1970-01-01 00:00:00");

graph.GetXaxis()->SetTimeDisplay(1);

graph.Draw("AL");

While the input data is invalid, a crash could be easily avoided by checking return value (demonstrated in https://github.com/tc3t/qoot/commit/51190613d6eae086702ff50830099b736baf8133).