Question about heap corruption --- see screen plot for detai

※为你歌唱※

I was writing my BigInt class to compute 500! The class uses dynamically allocated array as interal data member.
I got an error about "heap", which you can see from the attached .jpg file, although my program does give the correct answer for 500!:

12201368259911100687012387854230469262535743428031928421924135883858453731538819
97605496447502203281863013616477148203584163378722078177200480785205159329285477
90757193933060377296085908627042917454788242491272634430567017327076946106280231
04526442188787894657547771498634943677810376442740338273653974713864778784954384
89595537537990423241061271326984327745715546309977202781014561081188373709531016
35632443298702956389662891165897476957208792692887128178007026517450776841071962
43903943225364226052349458501299185715012487069615681416253590566934238130088562
49246891564126775654481886506593847951775360894005745238940335798476363944905313
06232374906644504882466507594673586207463792518420045936969298102226397195259719
09452178233317569345815085523328207628200234026269078983424517120062077146409794
56116127629145951237229913340169552363850942885592018727433795173014586357570828