lowering: Handle malformed ... expressions

[mlechu]

Fixes #51572. The expander had a looser definition of a vararg than the recursive logic destructing the vararg, so a bad expression like (call f (... x y)) could cause a stack overflow via indestructible mutant semi-vararg.

This change produces a saner error in this situation:

julia> macro foo(); Expr(:(...), 1, 2, 3); end; (@foo,)
ERROR: syntax: wrong number of expressions following "..." around REPL[1]:1
Stacktrace:
 [1] top-level scope
   @ REPL[1]:1