add assertions after memory alloc to make sure the alloc is successful.

[ken4647]

Add assertions after memory alloc for SetStringRaw, SetObjectRaw, SetArrayRaw and DoCopyMembers in document.h.

[tencent-adm]

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[tmorrisman]

Thank you!! Adding this helped to track down a problem that was otherwise a silent failure. Without this, the parse call showed no error but contained no DOM objects.