typeset: fix short integer restriction (#166)

This commit corrects how shortint was being applied to various
possible typeset variables in error. The short integer option
modifier 'typeset -s' should only be able to be applied if the
the variable is also an integer. Several issues were resolved
with this fix:
- 'typeset -s': created a short integer having an invalid base
  of zero. 'typeset -s foo' created 'typeset -s -i 0 foo=0' and
  now will result in an empty string.
- 'typeset -sL': previously resulted in a segmentation fault.

The following are the various incorrect 'typeset' instances
that have been fixed:

$ 'export foo; typeset -s foo; readonly foo; typeset -p foo'
(before) typeset -x -r -s -i 0 foo=0
( after) typeset -x -r foo

$ 'typeset -sL foo=1*2; typeset -p foo'
(before) Segmentation fault (core dumped)
( after) typeset -L 3 foo='1*2'

$ 'typeset -sR foo=1*2; typeset -p foo'
(before) typeset -s -i foo=2
( after) typeset -R 3 foo='1*2'

$ 'typeset -sZ foo=1*2; typeset -p foo'
(before) typeset -F 0 foo=2
( after) typeset -Z 3 -R 3 foo='1*2'

src/cmd/ksh93/bltins/typeset.c: b_typeset():
- Add conditional check within the 's' option to only
  apply NV_SHORT as well as remove any NV_LONG flag
  if NV_INTEGER flag was set.
- Relocate shortint conditional logic to the 'i' option.

src/cmd/ksh93/tests/attributes.sh:
- Adjust regression tests for '-s' and add '-si' check.

NEWS

@@ -5,6 +5,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 2021-02-01:
 
+- Fixed a bug in 'typeset': the '-s' modifier option for short integer will
+  now only be applied if the integer option '-i' is also present, avoiding
+  inconsistent results and a crash.
+  
 - Fixed: scalar arrays (-a) and associative arrays (-A) of a type created by
   'enum' allowed values not specified by the enum type, corrupting results.