gdbstub: Reject invalid RLE repeat counts

gdbstub: Reject invalid RLE repeat counts

"Debugging with GDB / Appendix E GDB Remote Serial Protocol /
Overview" specifies "The printable characters '#' and '$' or with a
numeric value greater than 126 must not be used."  gdb_read_byte()
only rejects values < 32.  This is wrong.  Impact depends on the caller:

* gdb_handlesig() passes a char.  Incorrectly accepts '#', '$' and
  '\127'.

* gdb_chr_receive() passes an uint8_t.  Additionally accepts
  characters with the most-significant bit set.

Correct the validity check to match the specification.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190514180311.16028-4-armbru@redhat.com>