Fix some sporadic segfaults that are triggered when SmallVector's heap

Fix some sporadic segfaults that are triggered when SmallVector's heap
storage lands near the end of the available address space. In the expression
Begin+N > Capacity, the Begin+N was overflowing. Fix this by replacing it
by with an expression that doesn't involve computation of an address
beyond the end of allocated memory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37171 91177308-0d34-0410-b5e6-96231b3b80d8