scp.c: improved command length calculation
scp.c: improved command length calculation

Reduced number of calls to strlen, because shell_quotearg already
returns the length of the resulting string (e.q. quoted path)
which we can add to the existing and known cmd_len.
Removed obsolete call to memset again, because we can put a final
NULL-byte at the end of the string using the calculated length.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2