Detect an OOM condition in the realpath() function of the fileio.c extension

Detect an OOM condition in the realpath() function of the fileio.c extension
and cause that function to return NULL.  To Do:  we should go back in and
fix realpath() to raise an SQLITE_NOMEM error on OOM rather than returning
NULL.  But we will delay that, in as much as OOMs are all but impossible
on modern OSes.
[bug:/info/2026-06-10T07:46:32Z|Bug 2026-06-10T07:46:32Z].

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