pallet-referenda: Detect incorrect pre-image length (#3850)

There has been a case that a referenda failed because the length given
to `submit` was incorrect. The pallet can actually check the length if
the pre-image already exists to ensure that these kind of issues are not
happening again.

prdoc/pr_3850.prdoc

@@ -0,0 +1,15 @@
+title: Detect incorrect pre-image length when submitting a referenda
+
+doc:
+  - audience: Runtime User
+    description: |
+      When submitting a referenda the `proposal` is passed as argument.
+      The `proposal` is most of the time a reference to a `pre-image` and
+      which also contains the length of the `pre-image`. This pull request
+      adds some logic to check that if the `pre-image` already exists and if
+      it exists, it ensures that the length is passed correctly. This prevents
+      that the referenda can not be executed because of a mismatch of this
+      length.
+
+crates:
+  - name: pallet-referenda