lensfun: allocate buffers for AddMount()/AddCompat() large enough

...so that they can hold the terminating '\0' in C strings.

Found with clang's -fsanitize=address while debugging
RawTherapee 5.8 crashers.

Later found to be already fixed upstream here:
https://github.com/lensfun/lensfun/commit/de954c952929316ea2ad0f6f1e336d9d8164ace0

Not handed in for maintainer approval because obviously correct,
maintainer will be notified though.