Fix two crashes related to kshdb (#82)

This commit fixes two different crashes related to kshdb:
- When redirect is given an invalid file descriptor, a segfault
  no longer occurs. Reproducer:
  $ ksh -c 'redirect 9>&200000000000'

- Fix a crash due to free(3) being used on an invalid pointer.
  This can be reproduced with kshdb (commands from att/ast#582):
  $ git clone https://github.com/rocky/kshdb.git
  $ cd kshdb
  $ ksh autogen.sh
  $ echo "print hi there" > $HOME/.kshdbrc
  $ ./kshdb -L . test/example/dbg-test1.sh

src/cmd/ksh93/bltins/misc.c: b_dot_cmd():
- The string pointed to by shp->st.filename must be able to be
  freed from memory with free(3), so duplicate the string with
  strdup(3).

src/cmd/ksh93/sh/io.c: sh_redirect():
- Show an error message when a file descriptor is invalid to
  fix a memory fault.

src/cmd/ksh93/tests/io.sh

@@ -563,5 +563,9 @@ result=$("$SHELL" -ic 'echo >(true) >/dev/null' 2>&1)
 "$SHELL" -c 'read -u-2000000' 2> /dev/null
 [[ $? == 1 ]] || err_exit "Negative file descriptors cause 'read -u' to crash"
 
+# An out of range fd shouldn't segfault with redirections
+"$SHELL" -c 'true 9>&20000000000000000000' 2> /dev/null
+[[ $? == 1 ]] || err_exit "Out of range file descriptors cause redirections to segfault"
+
 # ======
 exit $((Errors<125?Errors:125))