data parser BUGFIX negative values of unsigned integers
data parser BUGFIX negative values of unsigned integers

strtoull() handles negative values as valid input for unsigned integers
and converts them to equivalent unsigned values (UMAX - abs_value). For
YANG values this is an error and negative value is invalid.

Fixes #345

PavolVican/libyang,PavolVican/libyang,PavolVican/libyang,PavolVican/libyang,PavolVican/libyang