Turn Bob Moyer loose with fairy tales, frogs and puns, and you’re in for a treat. Reviewed by Robert P. Moyer GRETEL AND THE CASE OF THE MISSING FROG PRINTS. A Brothers Grimm Mystery. By P.J. Brackston. Pegasus Crime. 233 pages. $15.95, paperback. Sometimes the funniest thing about a joke is its setup. Sometimes the…