Fix StreamingMemoryObject to respect known object size.

The existing code for method StreamingMemoryObject.fetchToPos does not respect
the corresonding call to setKnownObjectSize(). As a result, it allows the
StreamingMemoryObject to read bytes past the object size.

This patch provides a test case, and code to fix the problem.

Patch by Karl Schimpf
Differential Revision: http://reviews.llvm.org/D8931

llvm-svn: 237939