Fix crashes caused by 'typeset -RF' (#47)

Variables created with 'typeset -RF' were being treated as
short integers, even though they are actually floating point
values. As a result the following example will cause a crash:

$ typeset -RF foo=1
$ test "$foo"

This is fixed by checking for 'NV_DOUBLE' with 'nv_isattr',
which prevents ksh from treating floating point values as
short integers due to '== NV_INT16P' excluding 'NV_DOUBLE'.
This bugfix was backported from ksh93v- 2013-10-10-alpha.

src/cmd/ksh93/sh/array.c,
src/cmd/ksh93/sh/name.c,
src/cmd/ksh93/sh/nvdisc:
- Avoid treating floating point values as short integers by
  checking for 'NV_DOUBLE' with 'nv_isattr'.

src/cmd/ksh93/tests/types.sh:
- Add a regression test for the 'typeset -RF' crash. The
  crash cannot be replicated if 'typeset -RF' sets 'foo'
  to zero.

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.
 
+2020-06-28:
+
+- Variables created with 'typeset -RF' no longer cause a memory fault
+  when accessed.
+
 2020-06-26:
 
 - Changing to a directory that has a name starting with a '.' will no