Fix is_multiple_of to account for 0

[WizardOfMenlo]

This pull request fixes the implementation to is_multiple_of to account for the case in which one of the operand is zero. The current implementation panics, while this one currently concludes that 0 is a multiple of 0 (and in fact the only one).

The change is a minimal check, and an inclusion of this edge case in the tests.

[cuviper]

Thanks!

bors r+

[bors[bot]]

Build succeeded:

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