The current code in `parse_section_header_ext` is only prepared to
properly handle out-of-memory conditions for the `git_buf` structure.
While very unlikely and probably caused by a programming error, it is
also possible to run into error conditions other than out-of-memory
previous to reaching the actual parsing loop. In these cases, we will
run into undefined behavior as the `rpos` variable is only initialized
after these triggerable errors, but we use it in the cleanup-routine.

Fix the issue by unifying the function's cleanup code with an
`end_error` section, which will not use the `rpos` variable.