free leaked schema string in GetFilename

[dxbjavid]

GetFilename hands the result of C.CString straight to sqlite3_db_filename, which only borrows the pointer, so the allocation is never released and a little C heap is leaked on every call. It is the only place in the package that builds a C string for an sqlite3 call without a matching free, the siblings all defer it. Stored the pointer and freed it the same way so the schema name no longer leaks.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.57%. Comparing base (18cdded) to head (a3cd5cd).
⚠️ Report is 100 commits behind head on master.

Files with missing lines	Patch %	Lines
sqlite3.go	0.00%	3 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1408      +/-   ##
==========================================
+ Coverage   47.16%   51.57%   +4.41%     
==========================================
  Files          12       13       +1     
  Lines        1533     1869     +336     
==========================================
+ Hits          723      964     +241     
- Misses        669      744      +75     
- Partials      141      161      +20     

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