[Issue 18904] New: core.internal.string has issues with radix

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18904

          Issue ID: 18904
           Summary: core.internal.string has issues with radix
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: schveiguy at yahoo.com

radices of 0 or 1 will cause crashes/infinite loops. radices greater than 36
will not be printable.

For the numDigits function, we can make sure it makes sense.

For the other functions, I'll just return a blank string, making the error
detectable elsewhere.

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