BumpPtrAllocator: Make sure threshold cannot be initialized with a value smaller than the slab size.

This replaces r151834 with a simpler fix.

llvm-svn: 151842
diff --git a/llvm/unittests/Support/AllocatorTest.cpp b/llvm/unittests/Support/AllocatorTest.cpp
index bc5bd3d..8b463c1 100644
--- a/llvm/unittests/Support/AllocatorTest.cpp
+++ b/llvm/unittests/Support/AllocatorTest.cpp
@@ -98,7 +98,7 @@
   BumpPtrAllocator Alloc(128);
 
   Alloc.Allocate(200, 0);
-  EXPECT_EQ(1U, Alloc.GetNumSlabs());
+  EXPECT_EQ(2U, Alloc.GetNumSlabs());
 }
 
 // Mock slab allocator that returns slabs aligned on 4096 bytes.  There is no