hi
My requirement is i have a file with some records like this

file name ::xyz

a=1
b=100,200
,300,400
,500,600
c=700,800
d=900

i want to change my file
a=1
b=100,200,300,400
c=700,800
d=900

if record starts with " , " that line should fallows the previous line.please give script or command to get my required output file

thanks