[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

(cherry picked from commit 51229ff7f4d3517706a1cdc1a2943ede1c605089)
Signed-off-by: Ankur Dave <ankurdave@gmail.com>
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

(cherry picked from commit dc30e4504abcda1774f5f09a08bba73d29a2898b)
Signed-off-by: Ankur Dave <ankurdave@gmail.com>