Fix Issue 21606 - pow(NaN,0) gives 1 not NaN

[berni44]

Detecting NaNs early speeds calculation up considerably when NaNs are involved (and removes a bug).

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21606	normal	pow(NaN,0) gives 1 not NaN

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7819"

[berni44]

Ooops. I just now realized, that the pow(float, float) explicitly makes an exception for NaN^^0. Internet research brought up this: https://stackoverflow.com/questions/17863619/why-does-nan0-1

Seems to be a weird convention...