[2020-01-05] - Found with American Fuzzy Lop

NULL pointer dereference in the SFD_AssignLookups() function, in sfd1.c.

Issue can be reproduced by running:

```
fontforge test04.sfd
```

```
=================================================================
==2985==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000028 (pc 0x7fb18974e2e3 bp 0x7ffdf9bc67c0 sp 0x7ffdf9bc6710 T0)
==2985==The signal is caused by a READ memory access.
==2985==Hint: address points to the zero page.
    #0 0x7fb18974e2e2 in SFD_AssignLookups /home/fcambus/fontforge-20190801/fontforge/sfd1.c:678
    #1 0x7fb1897ac2de in SFD_GetFont /home/fcambus/fontforge-20190801/fontforge/sfd.c:9011
    #2 0x7fb1897ace0f in SFD_Read /home/fcambus/fontforge-20190801/fontforge/sfd.c:9077
    #3 0x7fb1897ad360 in _SFDRead /home/fcambus/fontforge-20190801/fontforge/sfd.c:9110
    #4 0x7fb1898067bd in _ReadSplineFont /home/fcambus/fontforge-20190801/fontforge/splinefont.c:1178
    #5 0x7fb1898080bf in ReadSplineFont /home/fcambus/fontforge-20190801/fontforge/splinefont.c:1321
    #6 0x7fb189808591 in LoadSplineFont /home/fcambus/fontforge-20190801/fontforge/splinefont.c:1379
    #7 0x7fb1893c6bbc in ViewPostScriptFont /home/fcambus/fontforge-20190801/fontforge/fontviewbase.c:1347
    #8 0x7fb18b21589e in fontforge_main /home/fcambus/fontforge-20190801/fontforgeexe/startui.c:1392
    #9 0x560583aae1ec in main /home/fcambus/fontforge-20190801/fontforgeexe/main.c:33
    #10 0x7fb18a8a11e2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x271e2)
    #11 0x560583aae10d in _start (/home/fcambus/fontforge-20190801/fontforgeexe/.libs/fontforge+0x110d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/fcambus/fontforge-20190801/fontforge/sfd1.c:678 in SFD_AssignLookups
==2985==ABORTING
```