BUG: stats: When handling the arguments to beta_gen.fit, don't rely on np.log
to generate a warning when its argument is less than or equal to 0.
Fixes gh-2771.
BUG: stats: Raise a ValueError in beta_gen.fit if all the parameters are fixed.
Merge pull request #2801 from WarrenWeckesser/bug-beta-fit

BUG: stats: Fix a couple issues in the recently updated beta_gen.fit method.