[libFuzzer] Pass the current size to Mutate in MinimizeCrashLoop (#223470)

Fixes #149942. MinimizeCrashLoop passed U.size() to every MD.Mutate call
in the mutate_depth loop, so following mutations operated on a stale
size. Track the size across mutations, like MutateAndTestOne already
does.

GitOrigin-RevId: c7d4921b39bf5710968872213935198efbb310fd