gfileutils: Correctly reset start value when canonicalising paths

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
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