Fix segfault when calling git_blame_buffer()
This change makes sure that the hunk is not null before trying to
dereference it. This avoids segfaults, especially when blaming against a
modified buffer (i.e. the index).
Fixes: #5443
blame: use size_t for line counts in git_blame_hunk
It is not unreasonable to have versioned files with a line count
exceeding 2^16. Upon blaming such files we fail to correctly keep
track of the lines as `git_blame_hunk` stores them in `uint16_t`
fields.
Fix this by converting the line fields of `git_blame_hunk` to
`size_t`. Add test to verify behavior.