sed join lines together

 sed ':a;/0$/{N;s/\n//;ba}'

In a loop (branch ba to label :a), if the current line ends in 0 (/0$/) append next line (N) and remove inner newline (s/\n//).

awk:

awk '{while(/0$/) { getline a; $0=$0 a; sub(/\n/,_) }; print}'

Perl:

perl -pe '$_.=<>,s/\n// while /0$/'

bash:

while read line; do 
    if [ ${line: -1:1} != "0" ] ; then 
        echo $line
    else echo -n $line
fi
done

awk could be short too:

awk '!/0$/{printf $0}/0$/'

test:

kent$  cat t
#aasdfasdf
#asbbb0
#asf
#asdf0
#xxxxxx
#bar

kent$  awk '!/0$/{printf $0}/0$/' t
#aasdfasdf#asbbb0
#asf#asdf0
#xxxxxx#bar

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