weird error in thueinit

[Max Alekseyev <maxale@gmail.com>]

In my lengthy computations involving many thueinit() / thue() calls, I
suddenly get the following error:

  *** thueinit: incorrect priority in nfroots: variable x >= x

in the call "thueinit(p)" with apperently innocent p = -29*x^4 +
184*x^3 - 294*x^2 + 184*x - 29.
Right after this, calling thueinit(p) causes the same error, while
explicit calling thueinit(-29*x^4 + 184*x^3 - 294*x^2 + 184*x - 29)
works fine.
And after the latter call, restart of the whole computations runs
without any glitch.

I cannot reproduce the problem from scratch, it appears only inside
the computation.
Both release 2.5.4 and development 2.6.0 versions are susceptible.
I hope this information would help to nail the problem down.

Regards,
Max