`secp256k1_fe_equal` negates `a` before adding `b`.
That gives the temporary value magnitude 2, and the following field addition requires the input magnitudes to sum to at most 32.
So the largest `b` magnitude the implementation can accept is 30, not 31.
Lower the documented and checked bound for `b` to 30.
Adjust the focused test to use random field elements with randomized magnitudes within the accepted `a <= 1` and `b <= 30` bounds.
Co-authored-by: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>
Co-authored-by: Tim Ruffing <me@real-or-random.org>
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