Fix rare corner case in ntspawn.c

[jeromew]

There could be cases when libc/proc/describefds.c calls calloc(handlecount, ..) with handlecount = 0
to initialize what will later become opt_lpExplicitHandleList. But calloc(0, ..) does not always return null
according to https://learn.microsoft.com/ka-ge/cpp/c-runtime-library/reference/calloc?view=msvc-140

In the Microsoft implementation, if number or size is zero, calloc returns a pointer to an allocated block
of non-zero size. An attempt to read or write through the returned pointer leads to undefined behavior.

If the chosen conditional is a->opt_lpExplicitHandleList then UpdateProcThreadAttribute is called and
produces a Windows error. The modification avoids this problem.

[jart]

Good eye. LGTM. Please confirm the edit I made to your PR is correct, then I'll merge.

[jeromew]

LGTM. I should have spotted this. Thanks for the review

[jart]

Commit message edited to follow project style.

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