Fix crash due to double free() when sourcing multiple files

There is a bug in sh_eval() that may cause ksh to crash due to a
double free() after sourcing multiple files with '.' or 'source'
if a longjmp is triggered, e.g. by a syntax error.

This applies a fix from Siteshwar Vashist:
https://www.mail-archive.com/ast-developers@lists.research.att.com/msg01943.html

src/cmd/ksh93/sh/xec.c: sh_eval():
- Zero file descriptor io_save after closing it. This prevents a
  double free() after returning from a longjmp.

src/cmd/ksh93/tests/basic.sh:
- Add reproducer as regression test.

NEWS

@@ -3,6 +3,10 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-07-09:
+
+- Fixed a crash on syntax error when sourcing/dotting multiple files.
+
 2020-07-07:
 
 - Four of the date formats accepted by 'printf %()T' have had their