Ibeta inaccurate/incorrect for small a and b values when using single-precision floats

[mborland]

See: scipy/scipy#22682 for reproducer

[pearu]

Just FYI, the corresponding issue in JAX was fixed in jax-ml/jax#27107: see the code around comments " When a is close to zero.." and "For very small a and to avoid division by zero..".