bug#50720: unnamed &rest broken

[Mattias Engdegård]

The interpreter and compiler allow &rest to be used without a variable name 
following but the generated byte code is completely broken:

(funcall (byte-compile (lambda (&rest) 'ta)))

crashes, and

(defun boo (a &rest)
  (if a a (list 1 2 3 4)))

(boo 'hiss)
=> hiss        ; interpreted
=> (1 2 3 4)   ; compiled

The reason is that the compiler generates code from the argument variable list 
but the byte-code interpreter will only look at the signature code which was 
generated from the actual signature:

(byte-compile (lambda (&rest) 'ta))
=> #[128 "\300\207" [ta] 1 "..."]

The 128 indicates zero positional parameters and a &rest argument, and the 1 is 
the maximum stack size required which is wrong; 2 stack slots are needed and 
that's what we get if naming the argument:

(byte-compile (lambda (&rest _r) 'ta))
=> #[128 "\300\207" [ta] 2 "..."]

In the `boo` case above, it is clear that the compiler doesn't expect any &rest 
param to have been pushed at all so the stack offsets are wrong.

Now, either we fix this bug or we stop pretending that unnamed &rest arguments 
work at all and signal an error, because it's clear from the above that they 
can't have seen much use.