Typo

git-svn-id: http://svn.osgeo.org/postgis/trunk@12635 b70326c6-7e19-0410-871a-916f4a2858ee
Subtract size of offdb raster paths in test for ST_MemSize

Doing so should make the resulting size equal on all systems,
fixing the travis builds:
https://travis-ci.org/postgis/postgis#L2373-L2380

git-svn-id: http://svn.osgeo.org/postgis/trunk@12636 b70326c6-7e19-0410-871a-916f4a2858ee