The example MM font causes a crash if you try to interpolate it

[ctrlcctrlv]

I don't really think this matters, but my policy is if I find a crash I report it (we never know what might end up being important!), so I'm doing that.

1. Open CaslonMM.sfd
2. MM→Blend to New Font…
3. Press OK, accept defaults
4. Receive error
   
5. Press OK on error box
6. Press OK again on «Blend to New Font…» dialog which remains open
7. FontForge crashes
   

   Program received signal SIGSEGV, Segmentation fault.
   
   0x00001555550e6d76 in GradientFree (grad=0x660000006f) at fontforge/splineutil.c:5902

#0  0x00001555550e6d76 in GradientFree (grad=0x660000006f) at fontforge/splineutil.c:5902
#1  0x00001555550d785b in SCReinstanciateRefChar (sc=0x5555566bdd40, rf=0x555556797170, layer=1)
    at fontforge/splineutil.c:3071
#2  0x0000155554ee7923 in MMBlendChar (mm=0x555555e0e6e0, gid=27) at fontforge/mm.c:546
#3  0x0000155554ee81bb in MMReblend (fv=0x5555566e3df0, mm=0x555555e0e6e0) at fontforge/mm.c:648
#4  0x0000155554ee8ea3 in MMCreateBlendedFont
    (mm=0x555555e0e6e0, fv=0x5555566e3df0, blends=0x7fffffffb440, tonew=1) at fontforge/mm.c:798
#5  0x00005555557815a3 in MMCB_OK (g=0x5555568edf60, e=0x7fffffffb590) at fontforgeexe/mmdlg.c:509
#6  0x000055555584045d in GButtonInvoked (b=0x5555568edf60, ev=0x7fffffffb8b0) at gdraw/gbuttons.c:257
#7  0x00005555558413d4 in gbutton_mouse (g=0x5555568edf60, event=0x7fffffffb8b0) at gdraw/gbuttons.c:485
#8  0x000055555584b99c in _GWidget_Container_eh (gw=0x5555563f8b70, event=0x7fffffffb8b0)
    at gdraw/gcontainer.c:304
#9  0x000055555584d81c in _GWidget_TopLevel_eh (gw=0x5555563f8b70, event=0x7fffffffb8b0)
    at gdraw/gcontainer.c:747
#10 0x0000555555860524 in _GGDKDraw_CallEHChecked (gw=0x5555563f8b70, event=0x7fffffffb8b0, eh=
    0x55555584ce8e <_GWidget_TopLevel_eh>) at gdraw/ggdkdraw.c:329
#11 0x0000555555862f47 in _GGDKDraw_DispatchEvent (event=0x555555bfe9f0, data=0x555555c1aef0)
    at gdraw/ggdkdraw.c:1232
#12 0x00001555541bc8b4 in  () at /usr/lib/libgdk-3.so.0
#13 0x0000155554169904 in  () at /usr/lib/libgdk-3.so.0
#14 0x0000155553b4d3ae in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#15 0x0000155553b4f1c1 in  () at /usr/lib/libglib-2.0.so.0
#16 0x0000155553b4f201 in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#17 0x0000555555865bf6 in GGDKDrawProcessOneEvent (gdisp=0x555555c1aef0) at gdraw/ggdkdraw.c:2179
#18 0x000055555585169c in GDrawProcessOneEvent (gdisp=0x555555c1aef0) at gdraw/gdraw.c:769
#19 0x0000555555783d2f in MMChangeBlend (mm=0x555555e0e6e0, fv=0x555555e22bb0, tonew=1)
    at fontforgeexe/mmdlg.c:795
#20 0x00005555557010f2 in FVMenuBlendToNew
    (gw=0x5555562bf0d0, UNUSED_mi=0x5555563f93c0, UNUSED_e=0x7fffffffcd10) at fontforgeexe/fontview.c:3976
#21 0x000055555587ec0a in gmenu_mouse (m=0x5555562eac50, event=0x7fffffffcd10) at gdraw/gmenu.c:948
#22 0x00005555558806cd in gmenu_eh (w=0x5555562ead60, ge=0x7fffffffcd10) at gdraw/gmenu.c:1389
#23 0x0000555555860524 in _GGDKDraw_CallEHChecked (gw=0x5555562ead60, event=0x7fffffffcd10, eh=
    0x5555558805f4 <gmenu_eh>) at gdraw/ggdkdraw.c:329
#24 0x0000555555862f47 in _GGDKDraw_DispatchEvent (event=0x555555bfebd0, data=0x555555c1aef0)
    at gdraw/ggdkdraw.c:1232
#25 0x00001555541bc8b4 in  () at /usr/lib/libgdk-3.so.0
#26 0x0000155554169904 in  () at /usr/lib/libgdk-3.so.0
#27 0x0000155553b4d3ae in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#28 0x0000155553b4f1c1 in  () at /usr/lib/libglib-2.0.so.0
#29 0x0000155553b4f201 in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#30 0x0000555555865c72 in GGDKDrawEventLoop (gdisp=0x555555c1aef0) at gdraw/ggdkdraw.c:2190
#31 0x0000555555851779 in GDrawEventLoop (gdisp=0x555555c1aef0) at gdraw/gdraw.c:789
#32 0x0000555555815b29 in fontforge_main (argc=2, argv=0x7fffffffe148) at fontforgeexe/startui.c:1415
#33 0x00005555555aab29 in main (argc=2, argv=0x7fffffffe148) at fontforgeexe/main.c:33