core/conv: Only consider error for non-zero soft values

core/conv: Only consider error for non-zero soft values

If the input value is '0' it should not really affect the error
since it's just an indecisive bit. We accept this either an internal
'0' (generated via puncture) or as an external '0' (generated via an
external puncturing scheme). A real received bit should never be '0',
it's always gonna be closer to 1 or the other value ...

(thanks to mad@auth.se on the ML for the idea)

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>