hush: fix "redirects can close script fd" bug

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-misc/redir_script.tests

@@ -0,0 +1,29 @@
+# Builds a " 3>&- 4>&-" string.
+# Note: one of these fds is a directory opened to /proc/self/fd
+# for globbing. It is unwanted, but I don't know how to filter it out.
+find_fds() {
+	fds=""
+	for f in /proc/self/fd/*; do
+		test "$f" = "/proc/self/fd/0" && continue
+		test "$f" = "/proc/self/fd/1" && continue
+		test "$f" = "/proc/self/fd/2" && continue
+		fds="$fds ${f##*/}>&-"
+	done
+}
+
+find_fds
+fds1="$fds"
+
+# One of the fds is open to the script body
+# Close it while executing something.
+eval "find_fds $fds"
+
+# Shell should not lose that fd. Did it?
+find_fds
+test x"$fds1" = x"$fds" && { echo "Ok: script fd is not closed"; exit 0; }
+
+echo "Bug: script fd is closed"
+echo "fds1:$fds1"
+echo "fds2:$fds"
+exit 1
+