<rdar://problem/11800213>

Fixed a crasher in the "thread continue" code. There were many logic errors in the DoExecute function where thread index IDs were being used where the actual zero based thread index should have been used. This could cause crashes to happen since looking up a thread by index ID, when the zero based index of a thread should be used would return an empty thread shared pointer and cause a NULL deref.

llvm-svn: 159686