Infinite loop when opening a file or a new file.

[Thinker]

Inkscape would go into an infinite loop when it opens a file or a new file and start with an empty content. Steps to reproduce issue:
 1. run Inkscape in FreeBSD without any arguments.
 2. open a SVG file or a new file
 3. run into an infinite loop

It is a memory menagement issue. Inkscape::DocumentSubset::Relations::_doRemove() access content of a record, immediately, after erease the record. It is potential fault for any OS, but the code of invalid memory accessing is very close to the code to free the memory and the issue was hidden in most OS. But, for FreeBSD, its free() function clean content of free memory, so the issue is explosed.

This issue can be resolved by the patch attached.

[Krzysztof Kosinski (tweenk)]

Applied in r9045.