Removed zeroing th.th_task_state for master thread at start of nested parallel.

The th.th_task_state for the master thread at the start of a nested parallel
should not be zeroed in __kmp_allocate_team() because it is later put in the
stack of states in __kmp_fork_call() for further re-use after exiting the
nested region. It is zeroed after being put in the stack.

Differential Revision: http://reviews.llvm.org/D13702

llvm-svn: 250847