Fix memory leak in TestGDALVectorReader

This fixes a memory leak in the TestGDALVectorReader from the
string returned by ExpandDataFileName().

Change-Id: I76ed9267a82f11f8c3c534c0d1e04b4b040f4c8d
Merge topic 'fix-gdal-vector-reader-leak' into master

15866b7a Fix memory leak in TestGDALVectorReader