[SPARK-4249][GraphX]fix a problem of EdgePartitionBuilder in Graphx

at first srcIds is not initialized and are all 0. so we use edgeArray(0).srcId to currSrcId

Author: lianhuiwang <lianhuiwang09@gmail.com>

Closes #3138 from lianhuiwang/SPARK-4249 and squashes the following commits:

3f4e503 [lianhuiwang] fix a problem of EdgePartitionBuilder in Graphx

(cherry picked from commit d15c6e9dc2860bbe56e31ddf71218ccc6d5c841d)
Signed-off-by: Ankur Dave <ankurdave@gmail.com>