SPARK-1329: Create pid2vid with correct number of partitions

Each vertex partition is co-located with a pid2vid array created in
RoutingTable.scala. This array maps edge partition IDs to the list of
vertices in the current vertex partition that are mentioned by edges in
that partition. Therefore the pid2vid array should have one entry per
edge partition.

GraphX currently creates one entry per vertex partition, which is a bug
that leads to an ArrayIndexOutOfBoundsException when there are more edge
partitions than vertex partitions. This commit fixes the bug and adds a
test for this case.

Resolves SPARK-1329. Thanks to Daniel Darabos for reporting this bug.