[UB] Avoid a really broken call to realloc that would later result in

a bad call to memcpy.

When we only have a buffer from one of the two reparse calls, we can
just return that buffer rather than going through the realloc/memcpy
dance.

Found with UBsan.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243950 91177308-0d34-0410-b5e6-96231b3b80d8