Workaround to support older Rustc

Author: dhardy

rand_core/src/block.rs

@@ -187,7 +187,7 @@ where <R as BlockRngCore>::Results: AsRef<[u32]> + AsMut<[u32]>
         let read_u64 = |results: &[u32], index| {
             if cfg!(any(target_endian = "little")) {
                 // requires little-endian CPU
-                let ptr: *const u64 = results[index..=index+1].as_ptr() as *const u64;
+                let ptr: *const u64 = results[index..index+2].as_ptr() as *const u64;
                 unsafe { ptr::read_unaligned(ptr) }
             } else {
                 let x = u64::from(results[index]);