[SPARK-2981][GraphX] EdgePartition1D Int overflow

minor fix
detail is here: https://issues.apache.org/jira/browse/SPARK-2981

Author: Larry Xiao <xiaodi@sjtu.edu.cn>

Closes #1902 from larryxiao/2981 and squashes the following commits:

88059a2 [Larry Xiao] [SPARK-2981][GraphX] EdgePartition1D Int overflow