NULL pointer check and SCP fix

["Oliver Stöneberg" <oliverst@xxxxxxxxx>]

Hi,

attached is a patch with two small changes:
- added a missing NULL pointer check for strdup()  in client.c
- fixed result of ssh_scp_string_mode() to make SCP work

These are two local changes I need to have - would be nice, if those 
could be incorporated in the upcoming 0.6 release and in the 0.5 
branch since I am still running 0.5.x

Greetings and Thanks
Oliver

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