Allowing passing a null pointer to getrandom() when length is 0

The Linux kernel will handle a null pointer passed to 'getrandom'
without error, as long as the length is also 0. The `getrandom` crate
relies on this behavior: https://github.com/rust-random/getrandom/blob/ab44edf3c7af721a00e22648b6c811ccb559ba81/src/linux_android.rs#L26

Since it works fine on the actual kernel (and should continue to, due to
the kernel's backwards-compatibility guarantees), Miri should support it
as well.

src/shims/foreign_items.rs

@@ -293,14 +293,20 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 // is called if a `HashMap` is created the regular way (e.g. HashMap<K, V>).
                 match this.read_scalar(args[0])?.to_usize(this)? {
                     id if id == sys_getrandom => {
-                        let ptr = this.read_scalar(args[1])?.not_undef()?;
+                        let ptr = this.read_scalar(args[1])?;
                         let len = this.read_scalar(args[2])?.to_usize(this)?;
 
                         // The only supported flags are GRND_RANDOM and GRND_NONBLOCK,
                         // neither of which have any effect on our current PRNG
                         let _flags = this.read_scalar(args[3])?.to_i32()?;
 
-                        this.gen_random(len as usize, ptr)?;
+                        if len != 0 {
+                            // Linux allows passing a null pointer as
+                            // long as the length is also 0. THerefore,
+                            // we only call 'not_undef' if we
+                            // have a non-zero length.
+                            this.gen_random(len as usize, ptr.not_undef()?)?;
+                        }
                         this.write_scalar(Scalar::from_uint(len, dest.layout.size), dest)?;
                     }
                     id => {