encoding/asn1: check Bytes slice length in BitString.At

[TurboRx]

BitString.At documentation states that it returns 0 if the index is
out of range. However, it only checks i >= b.BitLength without verifying
that i/8 is within len(b.Bytes), causing a runtime panic when BitLength
exceeds the slice capacity or when Bytes is nil/empty.

Similarly, BitString.RightAlign now safely handles non-positive
BitLength values.

Fixes #81234

[gopherbot]

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