Over-shifting in a POSIX function should cause scripts to exit (#106)

The required longjmp used to terminate scripts was not being run
when over-shifting in a POSIX function with a redirection. This
caused scripts to continue after an error in the shift builtin,
which is incorrect since shift is a special builtin. The
interpreter is sent into an indeterminate state that causes
undefined behavior as well:
$ cat reproducer.ksh
some_func() {
   shift 10
}

for i in a b c d e f; do
  echo "read $i"
  [ "$i" != "c" ] && continue
  some_func 2>&1
  echo "$i = c"
done
$ ksh ./reproducer.ksh
read a
read b
read c
/tmp/k[2]: shift: 10: bad number
c = c
read d
/tmp/k[2]: shift: 10: bad number
d = c
read e
/tmp/k[2]: shift: 10: bad number
e = c
read f
/tmp/k[2]: shift: 10: bad number
f = c

src/cmd/ksh93/sh/xec.c: sh_exec():
- Do the necessary longjmp needed to terminate the script after
  over-shifting in a POSIX function when the function call has a
  redirection.

src/cmd/ksh93/tests/functions.sh:
- Add the over-shifting regression test from ksh93v- 2013-10-10-alpha.

Bug report and fix on the old mailing list:
https://www.mail-archive.com/ast-developers@lists.research.att.com/msg00732.html

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-08-05:
+
+- Fixed a bug that caused scripts to continue running after over-shifting
+  in a function when the function call had a redirection.
+
 2020-07-31:
 
 - Fixed a bug that caused multidimensional associative arrays to be created