[buffer] Fix out-buffer under memory-alloc failure

This was broken in July refactoring of the buffer, and exposed to
ReverseChainSingleSubstFormat1 in 3807061d634b60bd6235d6e1d8c47a034377f924

Fixes:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=38800
https://bugs.chromium.org/p/chromium/issues/detail?id=1303552
2 files changed