cgosqlite: fix memory leak in ExpandedSQL

[alisdair]

Updates tailscale/corp#34015

[fserb]

Randomly jumping in on this. Isn't this true for all strings SQLite returns? Should we fix SQL() above and the other cases in cgosqlite? Like column names and errors?

[alisdair]

No, this is a special case:

sqlite/cgosqlite/sqlite3.h

Lines 4565 to 4570 in 77567f4

	** ^The strings returned by sqlite3_sql(P) and sqlite3_normalized_sql(P)
	** are managed by SQLite and are automatically freed when the prepared
	** statement is finalized.
	** ^The string returned by sqlite3_expanded_sql(P), on the other hand,
	** is obtained from [sqlite3_malloc()] and must be freed by the application
	** by passing it to [sqlite3_free()].

[bradfitz]

I guess it's too tedious to make this look for leaks, eh?

(probably not visible to https://pkg.go.dev/runtime#ReadMemStats ...)

[alisdair]

Yeah, just too fiddly for me on a Sunday morning. I would like to try it tomorrow though.