[OpenMP] Fix tasking bug for decreasing hot team nthreads

The __kmp_execute_tasks_template() function reads the task_team and
current_task from the thread structure. There appears to be a pathological
timing where the number of threads in the hot team decreases and so a
thread is put in the pool via __kmp_free_thread(). It could be the case that:
1) A thread reads th_task_team into task_team local variables
       and is then interrupted by the OS
2) Master frees the thread and sets current task and task team to NULL
3) The thread reads current_task as NULL

When this happens, current_task is dereferenced and a segfault occurs.
This patch just checks for current_task to not be NULL as well.

Differential Revision: https://reviews.llvm.org/D50651

llvm-svn: 340632