The standalone driver for libgit2's fuzzing targets makes use of
functions from libgit2 itself. While this is totally fine to do, we need
to make sure to always have libgit2 initialized via `git_libgit2_init`
before we call out to any of these. While this happens in most cases as
we call `LLVMFuzzerInitialize`, which is provided by our fuzzers and
which right now always calls `git_libgit2_init`, one exception to this
rule is our error path when not enough arguments have been given. In
this case, we will call `git_vector_free_deep` without libgit2 having
been initialized. As we did not set up our allocation functions in that
case, this will lead to a segmentation fault.

Fix the issue by always initializing and shutting down libgit2 in the
standalone driver. Note that we cannot let this replace the
initialization in `LLVMFuzzerInitialize`, as it is required when using
the "real" fuzzers by LLVM without our standalone driver. It's no
problem to call the initialization and deinitialization functions
multiple times, though.