Don't assert when given an empty range.

Don't assert when given an empty range.

llvm::RawMemoryObject handles empty ranges just fine, and the assert can
be triggered in the wild by e.g. invoking clang with a file that
included an empty pre-compiled header file when clang has been built
with assertions enabled. Without assertions enabled, clang will properly
report that the empty file is not a valid PCH.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158769 91177308-0d34-0410-b5e6-96231b3b80d8