nutdec: always check the get_str return value

If it fails, the buffers can be (partially) uninitialized.

This fixes 'Conditional jump or move depends on uninitialised value(s)'
valgrind warnings.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
