MDEV-38669: ASAN fail in Item_func_collect::add during ST_COLLECT

[DaveGosselin-MariaDB]

st_collect read past the end of its input because it assumed that the input was a valid geometry. Check that the length of the input is at least long enough to possibly contain a valid geometry.

[gemini-code-assist]

Code Review

This pull request addresses MDEV-38669, where ST_COLLECT could read past the end of a non-geometry argument, by introducing a helper method Geometry::is_valid_geometry_length to validate geometry buffer lengths. However, the review highlights an inconsistency: short invalid geometries are silently ignored during aggregation, whereas longer invalid geometries pass the initial length check and cause the aggregate to fail later. It is recommended to handle all invalid geometries consistently and add mixed-validity test cases.

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