Bug 7064: yang-to-source error on description

Existing logic of formatToParagraph() method doesn't expect a case
when very first text sequence is longer than 80 characters
and crashes in wrong condition branch. This has been fixed and tested
for various inputs.

Change-Id: Iefeb2b4063335561f3277c3ee40b3d9a46c418dc
Signed-off-by: Martin Ciglan <mciglan@cisco.com>