PUD: nmealib: fix parsing of date in GPRMC
PUD: nmealib: fix parsing of date in GPRMC

It appears that the field is optional/can be empty and that parsing
such an empty field would fail.

Signed-off-by: Ferry Huberts <1f083df4a0371a8eba2ce71fc6b0e214b2551710@mindef.nl>

nolith/olsrd,nolith/olsrd,diogomg/olsrd,zioproto/olsrd,sebkur/olsrd,ninuxorg/olsrd,diogomg/olsrd,sebkur/olsrd,cholin/olsrd,ninuxorg/olsrd,cholin/olsrd,ninuxorg/olsrd,nolith/olsrd,zioproto/olsrd,diogomg/olsrd,diogomg/olsrd-binary-heap,ninuxorg/olsrd,diogomg/olsrd-binary-heap,diogomg/olsrd,tdz/olsrd,diogomg/olsrd-binary-heap,tdz/olsrd,tdz/olsrd,diogomg/olsrd,diogomg/olsrd-binary-heap,sebkur/olsrd,sebkur/olsrd,diogomg/olsrd-binary-heap,zioproto/olsrd,sebkur/olsrd,diogomg/olsrd,zioproto/olsrd,tdz/olsrd,cholin/olsrd,sebkur/olsrd,tdz/olsrd,nolith/olsrd,diogomg/olsrd,cholin/olsrd,nolith/olsrd,zioproto/olsrd,cholin/olsrd,diogomg/olsrd-binary-heap,ninuxorg/olsrd,diogomg/olsrd-binary-heap