find: match nothing when -mindepth exceeds -maxdepth - #854

Fixes #778.

Problem

$ mkdir -p /tmp/dt/a/b/c && touch /tmp/dt/a/b/c/f /tmp/dt/a/x

$ /usr/bin/find /tmp/dt -mindepth 3 -maxdepth 1     # GNU
$ find /tmp/dt -mindepth 3 -maxdepth 1              # uutils, before
/tmp/dt/a

An empty depth range should match nothing, but uutils printed entries at depth 1.

Cause

walkdir's min_depth() and max_depth() setters both clamp min_depth down to max_depth, so -mindepth 3 -maxdepth 1 was silently turned into -mindepth 1 -maxdepth 1. Swapping the call order does not help — either setter clamps.

Fix

Detect the impossible range before building the walker. In that case walk the starting point only (max_depth(0)), so errors for a bad starting point are still reported, and discard every entry the walk yields.

$ find /nope -mindepth 3 -maxdepth 1
Error: /nope: No such file or directory (os error 2)   # rc=1, matching GNU's diagnostic

Verification

Every combination below now agrees with GNU find on output and exit code, with and without -depth:

Added find_mindepth_greater_than_maxdepth, covering both traversal orders. cargo test, cargo clippy --all-targets -- -D warnings and cargo fmt --check are all clean.

No GNU source was consulted; the fix is derived from walkdir's documented clamping behaviour.