Problem with factorial function

[DomHeadroom]

Describe the bug
If I try to do the factorial of a negative number instead of giving me Nan as result it give the factorial of the number as it was a positive number but negative, for example if try to do -5! it give me -120.

And I also wanted to report that instead of giving Nan as it's currently written in the code 0! should give as a result 1.
https://www.thoughtco.com/why-does-zero-factorial-equal-one-3126598

To Reproduce
Steps to reproduce the behavior:

1. Press the minus button
2. Press a random number
3. Then click the factorial button

Expected behavior
-5! should give me as a result Nan

Screenshots/videos
https://media.discordapp.net/attachments/831525884069675067/1076231040437342319/IMG_20230217_205730.jpg

Smartphone (please complete the following information):

• Device: Poco x3 nfc
• OS: Android 12, miui 13.0.4
• Version: 2.0.0 from f-droid
• Language: Italian

[Darkempire78]

-5! is like -(5!), but you are right, (-5)! gives -120 instead of NaN

[DomHeadroom]

But it does this in any case even if the -5 is a result of a previous operation

[lefterisgar]

Also if you type (5)! it will give you 5. Probably related. For some reason, when there are parentheses before the ! symbol, the code doesn't reach the factorial function.

[Darkempire78]

I fixed factoriel of 0

[Darkempire78]

Done