RASTER: fix gh54702 crash on wcs bad layers

[elpaso]

Fixes a crash on bad GDAL raster layers when the user chooses to keep it.

Fix #54702

[nyalldawson]

@elpaso low risk crash fixes like these should skip the queued branch 👍

[qgis-bot]

The backport to release-3_28 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 5d18be498d... Test for isse #54702
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

stdout

Auto-merging src/providers/wcs/qgswcsprovider.cpp
[backport-54707-to-release-3_28 40b9d44705] Fix crash on WCS bad layers
 Author: Alessandro Pasotti <elpaso@itopen.it>
 Date: Fri Sep 22 14:07:46 2023 +0200
 2 files changed, 6 insertions(+), 1 deletion(-)
Auto-merging tests/src/python/test_provider_gdal.py
CONFLICT (content): Merge conflict in tests/src/python/test_provider_gdal.py

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_28 release-3_28
# Navigate to the new working tree
cd .worktrees/backport-release-3_28
# Create a new branch
git switch --create backport-54707-to-release-3_28
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 4c36f297deaf9f31b702aed99df3af450c44055c,5d18be498dddb44c519fb2a3f444edc17b3672c1
# Push it to GitHub
git push --set-upstream origin backport-54707-to-release-3_28
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_28

Then, create a pull request where the base branch is release-3_28 and the compare/head branch is backport-54707-to-release-3_28.