Fix potential memory fault in ST_MakeValid

The fault would be triggered by passing a non-empty collection
to ST_MakeValid where any "made-valid" component would result
being non accepted by the final geometry collection constructor.

I'm not sure an input exists right now to trigger that case,
but worth fixing anyway.

Bug found by Alvaro Huarte:
https://github.com/postgis/postgis/pull/21

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12667 b70326c6-7e19-0410-871a-916f4a2858ee