Fix leak introduced in r194610, found by LSan.

LSan folks: LSan pointed to
    #0 0x4953e0 in operator new[](unsigned long) llvm/projects/compiler-rt/lib/asan/asan_new_delete.cc:64
    #1 0x7fb82af5372f in clang::RewriteRope::MakeRopeString(char const*, char const*) llvm/tools/clang/lib/Rewrite/Core/RewriteRope.cpp:796
instead of to the actual leak, which made tracking this down slower than
it could have been.