If a path starts with more than two slashes, the `start` value was
previously incorrect:
1. As per the `g_path_skip_root()` call, `start` was set to point to
after the final initial slash. For a path with three initial
slashes, this is the character after the third slash.
2. The canonicalisation loop to find the first dir separator sets
`output` to point to the character after the first slash (and it
overwrites the first slash to be `G_DIR_SEPARATOR`).
3. At this point, with a string `///usr`, `output` points to the second
`/`; and `start` points to the `u`. This is incorrect, as `start`
should point to the starting character for output, as per the
original call to `g_path_skip_root()`.
4. For paths which subsequently include a `..`, this results in the
`output > start` check in the `..` loop below not skipping all the
characters of a preceding path component, which is then caught by
the `G_IS_DIR_SEPARATOR (output[-1])` assertion.
Fix this by resetting `start` to `output` after finding the final slash
to keep in the output, but before starting the main parsing loop.
Relatedly, split `start` into two variables: `after_root` and
`output_start`, since the variable actually has two roles in the two
parts of the function.
Includes a test.
This commit is heavily based on suggestions by Sebastian Wilhemi and
Sebastian Dröge.
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
oss-fuzz#41563