rdp: Check for non-digits and errno in strtol call

Improve error checking for situations like RDP_FD=42foo, or where the
provided number is out of range.

Suggestion by Yong Bakos.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
1 file changed