Rollup merge of rust-lang#5784 - matthiaskrgr:ice_5780, r=phansch

Fix out of bounds access by checking length equality BEFORE accessing by index.

Fixes rust-lang#5780

changelog: fix out of bounds access in unnested_or_patterns lint.

Edit: I did not bother reducing a testcase from `librustc_typeck` crate but I can confirm that with the change the crash no longer occurs.

Author: flip1995

clippy_lints/src/unnested_or_patterns.rs

@@ -400,8 +400,8 @@ fn extend_with_matching(
 
 /// Are the patterns in `ps1` and `ps2` equal save for `ps1[idx]` compared to `ps2[idx]`?
 fn eq_pre_post(ps1: &[P<Pat>], ps2: &[P<Pat>], idx: usize) -> bool {
-    ps1[idx].is_rest() == ps2[idx].is_rest() // Avoid `[x, ..] | [x, 0]` => `[x, .. | 0]`.
-        && ps1.len() == ps2.len()
+    ps1.len() == ps2.len()
+        && ps1[idx].is_rest() == ps2[idx].is_rest() // Avoid `[x, ..] | [x, 0]` => `[x, .. | 0]`.
         && over(&ps1[..idx], &ps2[..idx], |l, r| eq_pat(l, r))
         && over(&ps1[idx + 1..], &ps2[idx + 1..], |l, r| eq_pat(l, r))
 }