Currently, `ecnist`'s `toBytes` and `getBytes` methods operate only on
properly initialized keys. If an un-initialized key is given, an
`IndexError` may be raised if the key's `xlen` / `qlen` property is
larger than the maximum buffer size. This patch hardens those functions
to report a proper error in that case.
Note that the library functions called by `init` and `initRaw` already
reject data that does not have the expected length, so these new checks
should not be reachable in practice.
2373ee0061