crash when gradients dependencies are bad

[hmil]

When selecting an item which has a broken gradient background inheritance chain, Inskape would crash.

Repro:
- Save this as weapons.svg and open it in Inkscape: http://pastebin.com/wLCrG1hh
- Open the fill&stroke panel
- Select the circle. Inkscape crashes

Here's a trace: http://paste.debian.net/plain/171640

I also found this piece of code where the return value of chase_hrefs is not checked in getVector but is checked in all other functions using chase_hrefs... (leading to the segfault seen in the trace) (in sp-gradient.cpp) http://pastebin.com/u8vea1ur

Note: The "weapons.svg" file was generated by Inkscape and was not tempered by an external tool (this could be a topic for another issue if someone manages to reproduce it, I didn't).

Inkscape Version (from file):
   inkscape:version="0.91 r13725"

[su_v (suv-lp)]

Reproduced (as discussed on irc):
* on Ubuntu 14.04 with Inkscape 0.91 and 0.91+devel;
* on OS X 10.7.5 with Inkscape 0.48.x, 0.91 and 0.91+devel r14109

--
<off-topic>Please always include information about OS/platform and Inkscape version in the description when filing new reports, thank you</off-topic>

[jazzynico (jazzynico)]

Also reproduced on Windows XP, Inkscape trunk rev. 14109, same trace.

[su_v (suv-lp)]

Not reproduced with Inkscape 0.91 r13841 (HEAD of current stable release branch) and Inkscape 0.91+devel rev 14411.

Bases on tests with archived trunk builds on OS X:
- reproduced with Inkscape 0.91+devel rev <= 14322,
- not reproduced with Inkscape 0.91+devel rev <= 14325;
this crashno longer happens after the changes in rev 14324:
Revision 14324: Prevent crash when "vector" doesn't exist (i.e. for mesh gradient).
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/14324

Revision 14324 was backported to the stable release branch in rev 13811.