Handle "EXTRA SPELL" magic effect

I made a spell that contained the effect "ExtraSpell". (What does that do by the way?) Anyhow, on casting it an exception is thrown:

#0  0x00007ffff11e5090 in __cxa_throw () from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/libstdc++.so.6
#1  0x0000000001186065 in ESM::MagicEffect::effectIdToString (effectID=126) at /home/xxx/tmp/openmw/openmw/components/esm/loadmgef.cpp:527
#2  0x0000000000d7e32a in MWGui::SpellIcons::updateWidgets (this=0x6210db0, parent=0x623b420, adjustSize=true)
    at /home/xxx/tmp/openmw/openmw/apps/openmw/mwgui/spellicons.cpp:150
#3  0x0000000000d1ac35 in MWGui::HUD::update (this=0x62149a0) at /home/xxx/tmp/openmw/openmw/apps/openmw/mwgui/hud.cpp:629
#4  0x0000000000c8c79a in MWGui::WindowManager::update (this=0x22b5950) at /home/xxx/tmp/openmw/openmw/apps/openmw/mwgui/windowmanagerimp.cpp:469
#5  0x00000000010d3fcd in OMW::Engine::frame (this=0x1849490, frametime=0.0182799995) at /home/xxx/tmp/openmw/openmw/apps/openmw/engine.cpp:174
#6  0x00000000010d8e72 in OMW::Engine::go (this=0x1849490) at /home/xxx/tmp/openmw/openmw/apps/openmw/engine.cpp:691
#7  0x00000000010c1d7c in main (argc=1, argv=0x7fffffffdc88) at /home/xxx/tmp/openmw/openmw/apps/openmw/main.cpp:386
#0  0x00007ffff11e5090 in __cxa_throw () from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/libstdc++.so.6

(RM-2906 from redmine: created on 2015-09-07 by Who Knows, , closed on 2018-05-13 by Andrei Kortunov)