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/sh/io.c

@@ -1194,7 +1194,7 @@ int	sh_redirect(Shell_t *shp,struct ionod *iop, int flag)
 						toclose = dupfd;
 						number++;
 					}
-					if(*number || dupfd > IOUFD)
+					if(*number || !sh_iovalidfd(shp,dupfd) || dupfd > IOUFD)
 					{
 						message = e_file;
 						goto fail;