sf-sparse-vector.cc bug, in Init function

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Make all_test, then find an error occured white testing sf-sparse-vector_test, 
assertion assert(x1.GetGroupId() == "2"); failed at line 27 of file 
sf-sparse-vector_test.cc.

Solution. Add a line "group_id_c_string[end - position]=0;" in 
sf-sparse-vector.cc line 145. cause string generated by strncpy is not always 
'\0' terminated.

Original issue reported on code.google.com by starshin...@gmail.com on 5 Nov 2012 at 4:22