Do not let the API object for the assembled grid live on

[PaulWessel]

GMT_Read_Data assigns a new object number to what is read, but if input file is a set of tiles it calls gmt_assemble_grid which also creates an object to pass from grdblend. However, once we get it back from grdblend we only want to keep the grid and discard the container since GMT_Read_Data already has a container for it.

gmtmex('grdcut @earth_relief_01m -R-37.0/-35.0/-2.3/-1.0')

works when built with this PR.

[github-actions]

The backport to 6.1 failed:

The process 'git' failed with exit code 1

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-6.1 6.1
# Navigate to the new working tree
cd .worktrees/backport-6.1
# Create a new branch
git switch --create backport-3771-to-6.1
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 46ff22613bbbc181bb4bf5b9ad4ebc928c1bab5f
# Push it to GitHub
git push --set-upstream origin backport-3771-to-6.1
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-6.1

Then, create a pull request where the base branch is 6.1 and the compare/head branch is backport-3771-to-6.1.

[PaulWessel]

Pinging @seisman

[seisman]

OK, will see what happened later.

[seisman]

So there is a conflict in src/gmt_internal.h

++<<<<<<< HEAD
++=======
+ EXTERN_MSC int gmtlib_delete_virtualfile  (void *API, const char *string);
+ EXTERN_MSC bool gmtlib_file_lock (struct GMT_CTRL *GMT, int fd);
+ EXTERN_MSC bool gmtlib_file_unlock (struct GMT_CTRL *GMT, int fd);
++>>>>>>> 46ff22613b... Do not let the API object for the assembled grid live one (#3771)

These two functions were added in PR #3735, but not backported to 6.1 branch. So they shouldn't be added to 6.1.

+ EXTERN_MSC bool gmtlib_file_lock (struct GMT_CTRL *GMT, int fd);
+ EXTERN_MSC bool gmtlib_file_unlock (struct GMT_CTRL *GMT, int fd);

[PaulWessel]

yes, correct.