[spec] Add note about control stack invariant to algorithm (WebAssembly#1498)

Author: rossberg

document/core/appendix/algorithm.rst

@@ -240,6 +240,8 @@ The type of the :ref:`label <syntax-label>` associated with a control frame is e
 
 Finally, the current frame can be marked as unreachable.
 In that case, all existing operand types are purged from the value stack, in order to allow for the :ref:`stack-polymorphism <polymorphism>` logic in :code:`pop_val` to take effect.
+Because every function has an implicit outermost label that corresponds to an implicit block frame,
+it is an invariant of the validation algorithm that there always is at least one frame on the control stack when validating an instruction, and hence, `ctrls[0]` is always defined.
 
 .. note::
    Even with the unreachable flag set, consecutive operands are still pushed to and popped from the operand stack.