reduce slice::panic_safe test size further in Miri

Author: RalfJung

library/alloc/tests/slice.rs

@@ -1722,7 +1722,7 @@ fn panic_safe() {
     let mut rng = thread_rng();
 
     // Miri is too slow
-    let lens = if cfg!(miri) { (1..10).chain(20..21) } else { (1..20).chain(70..MAX_LEN) };
+    let lens = if cfg!(miri) { (1..10).chain(0..0) } else { (1..20).chain(70..MAX_LEN) };
     let moduli: &[u32] = if cfg!(miri) { &[5] } else { &[5, 20, 50] };
 
     for len in lens {