[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>