misc: Check that get_slice() returned a big enough slice

This should be guaranteed by GuestMemory and GuestMemoryRegion, but
those traits are currently safe, so add checks to guard against
incorrect implementations of them.

Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>

block/src/lib.rs

@@ -440,8 +440,9 @@ impl Request {
 
             let origin_ptr = mem
                 .get_slice(data_addr, data_len)
-                .map_err(ExecuteError::GetHostAddress)?
-                .ptr_guard();
+                .map_err(ExecuteError::GetHostAddress)?;
+            assert!(origin_ptr.len() >= data_len);
+            let origin_ptr = origin_ptr.ptr_guard();
 
             // Verify the buffer alignment.
             // In case it's not properly aligned, an intermediate buffer is