Fix file descriptor leaks in the hist builtin (#336)

This commit fixes two file descriptor leaks in the hist built-in.
The bugfix for the first file descriptor leak was backported from
ksh2020. See:
https://github.com/att/ast/issues/872
73bd61b5

Reproducer:
  $ echo no
  $ hist -s no=yes

The second file descriptor leak occurs after a substitution error
in the hist built-in (this leak wasn't fixed in ksh2020).
Reproducer:
  $ echo no
  $ ls /proc/$$/fd
  $ hist -s no=yes
  $ hist -s no=yes
  $ ls /proc/$$/fd

src/cmd/ksh93/bltins/hist.c:
- Close leftover file descriptors when an error occurs and after
  'hist -s' runs a command.

src/cmd/ksh93/tests/builtins.sh:
- Add two regression tests for both of the file descriptor leaks.

src/cmd/ksh93/tests/builtins.sh

@@ -1273,5 +1273,30 @@ got=$(	readonly v=foo
 [[ $got == "$exp" ]] || err_exit "prefixing special builtin with 'command' does not stop it from exiting the shell on error" \
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 
+# ======
+# https://github.com/att/ast/issues/872
+hist_leak=$tmp/hist_leak.sh
+print 'ulimit -n 15' > "$hist_leak"
+for ((i=0; i!=11; i++)) do
+	print 'true foo\nhist -s foo=bar 2> /dev/null' >> "$hist_leak"
+done
+print 'print OK' >> "$hist_leak"
+exp="OK"
+got="$($SHELL -i "$hist_leak" 2>&1)"
+[[ $exp == "$got" ]] || err_exit "file descriptor leak in hist builtin" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
+# File descriptor leak after hist builtin substitution error
+hist_error_leak=$tmp/hist_error_leak.sh
+print 'ulimit -n 15' > "$hist_error_leak"
+for ((i=0; i!=11; i++)) do
+	print 'hist -s no=yes 2> /dev/null' >> "$hist_error_leak"
+done
+print 'print OK' >> "$hist_error_leak"
+exp="OK"
+got="$($SHELL -i "$hist_error_leak" 2>&1)"
+[[ $exp == "$got" ]] || err_exit "file descriptor leak after substitution error in hist builtin" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))