debug: fix argcheck diagnostics on Lua 5.1.

Lua 5.2 seems to treat `return argerror` (where argerror never
returns anyway) as subject to some kind of optimisation that loses
a call stack level by the time control flow reaches the `error`
call inside. Removing the `return` results in the same call stack
frame reference in 5.2 and 5.1, but requires incrementing the
level for the correct result.
* lib/std/debug.lua (argcheck): Drop the extraneous `return`
statement, and increment `level` before calling `argerror`.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>