eval_sub followed dangling pointer when debugging

Problem reported by Pip Cet (Bug#21245).
This bug could occur in eval_sub if the C compiler reused
storage associated with the ‘argvals’ local after ‘argvals’
went out of scope, and if the Elisp debugger stopped on Elisp
function exit and accessed ‘argvals’.  It could also occur if
a variadic function was called with so many arguments (over
2048 args on x86-64) that SAFE_ALLOCA_LISP called malloc, then
SAFE_FREE freed the arguments, then the memory manager used
the storage for other purposes, then the debugger accessed the
arguments.
* src/eval.c (eval_sub): Declare ‘argvals’ at top level of
function body.	Simplify local decls.
When allocating args via SAFE_ALLOCA, call
debugger before invoking SAFE_FREE, as the debugger needs
access to the args.
(eval_sub, apply_lambda): Rework to avoid need for
set_backtrace_debug_on_exit hack.  This is cleaner,
and should work better with buggy custom debuggers.

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