Make the number of physical CPU cores detection more robust

[PaulBone]

Bad data in /proc/cpuinfo can cause this calculation to divide by zero. While there's clearly a bug in my kernel causing a weird /proc/cpuinfo it's a good idea to make zstd robust to bad/weird data like this.

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[senhuang42]

Thanks for the PR @PaulBone! Looks like you just have to sign the CLA.

[PaulBone]

Thanks for the PR @PaulBone! Looks like you just have to sign the CLA.

Hi @senhuang42
My colleague Daniel Nazer dnazer@mozilla.com sent a message to cla@fb.com to add me to Mozilla's CLA agreement.

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