[graphX] GraphOps: random pick vertex bug

When `numVertices > 50`, probability is set to 0. This would cause infinite loop.

Author: yingjieMiao <yingjie@42go.com>

Closes #2553 from yingjieMiao/graphx and squashes the following commits:

6adf3c8 [yingjieMiao] [graphX] GraphOps: random pick vertex bug
Fixed the condition in StronglyConnectedComponents Issue: SPARK-3635

Author: oded <oded@HP-DV6.c4internal.c4-security.com>

Closes #2486 from odedz/master and squashes the following commits:

dd7890a [oded] Fixed the condition in StronglyConnectedComponents Issue: SPARK-3635
Minor cleanup of code.

Author: Reynold Xin <rxin@apache.org>

Closes #2581 from rxin/minor-cleanup and squashes the following commits:

736a91b [Reynold Xin] Minor cleanup of code.