Fix a crash when 'read -u' is given an invalid fd (#53)

File descriptors that are too far out of range will cause the
read builtin to crash. The following example will generate
two crashes:

$ ksh -c 'read -u 2000000' || ksh -c 'read -u-2000000'

The fix is to error out when the given file descriptor is out
of range. This bugfix is from Tomas Klacko, although it was
modified to use 'sh_iovalidfd' and reject numbers greater
than 'INT_MAX':
https://www.mail-archive.com/ast-developers@lists.research.att.com/msg01912.html
The question about 'shp->fdstatus[-1]' only applies to ksh93v-
(ksh93u+ doesn't have any references to 'shp->fdstatus[-1]').

src/cmd/ksh93/bltins/read.c:
- File descriptors that are out of range should be rejected
  with an error message (like invalid file descriptors that
  are in range). The seemingly redundant check for negative
  numbers is there because out of range negative numbers also
  cause memory faults despite the later 'fd<0' check.

src/cmd/ksh93/tests/io.sh:
- Add three tests for attempting 'read -u' on various invalid
  file descriptor numbers.

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-06-28"
+#define SH_RELEASE	"93u+m 2020-06-30"