[Issue 18434] BigInt gcd asserts when one argument is zero.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18434

hsteoh at quickfur.ath.cx changed:

           What    |Removed                     |Added
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            Summary|Failing case of BigInt gcd  |BigInt gcd asserts when one
                   |                            |argument is zero.

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