[2020-01-06] - Found with American Fuzzy Lop

NULL pointer dereference in the dumpcffnames() function, in tottf.c.

Issue can be reproduced by running:

```
fontforge -lang ff -c 'Open("test06.sfd"); Generate("test06.otf")'
```

```
=================================================================
==14023==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7f5624395ba5 bp 0x7ffda6285000 sp 0x7ffda6284778 T0)
==14023==The signal is caused by a READ memory access.
==14023==Hint: address points to the zero page.
    #0 0x7f5624395ba4  (/lib/x86_64-linux-gnu/libc.so.6+0x18bba4)
    #1 0x7f5624f8b8fb  (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x678fb)
    #2 0x7f56233f3a1b in dumpcffnames /home/fcambus/fontforge-20190801/fontforge/tottf.c:1795
    #3 0x7f56233ffb50 in dumptype2glyphs /home/fcambus/fontforge-20190801/fontforge/tottf.c:2634
    #4 0x7f562342907c in initTables /home/fcambus/fontforge-20190801/fontforge/tottf.c:5750
    #5 0x7f562342d475 in _WriteTTFFont /home/fcambus/fontforge-20190801/fontforge/tottf.c:6143
    #6 0x7f562342d611 in WriteTTFFont /home/fcambus/fontforge-20190801/fontforge/tottf.c:6171
    #7 0x7f5622fe00b4 in _DoSave /home/fcambus/fontforge-20190801/fontforge/savefont.c:845
    #8 0x7f5622fe2f7b in GenerateScript /home/fcambus/fontforge-20190801/fontforge/savefont.c:1269
    #9 0x7f5622ff99c4 in bGenerate /home/fcambus/fontforge-20190801/fontforge/scripting.c:2061
    #10 0x7f56230576bb in docall /home/fcambus/fontforge-20190801/fontforge/scripting.c:9632
    #11 0x7f562305855e in handlename /home/fcambus/fontforge-20190801/fontforge/scripting.c:9745
    #12 0x7f562305c8d0 in term /home/fcambus/fontforge-20190801/fontforge/scripting.c:9983
    #13 0x7f562305e1de in mul /home/fcambus/fontforge-20190801/fontforge/scripting.c:10128
    #14 0x7f562305eade in add /home/fcambus/fontforge-20190801/fontforge/scripting.c:10174
    #15 0x7f562305f9e7 in comp /home/fcambus/fontforge-20190801/fontforge/scripting.c:10249
    #16 0x7f562306044d in _and /home/fcambus/fontforge-20190801/fontforge/scripting.c:10293
    #17 0x7f5623060a79 in _or /home/fcambus/fontforge-20190801/fontforge/scripting.c:10325
    #18 0x7f5623061167 in assign /home/fcambus/fontforge-20190801/fontforge/scripting.c:10358
    #19 0x7f562306284f in expr /home/fcambus/fontforge-20190801/fontforge/scripting.c:10436
    #20 0x7f56230644de in ff_statement /home/fcambus/fontforge-20190801/fontforge/scripting.c:10649
    #21 0x7f5623065b92 in ProcessNativeScript /home/fcambus/fontforge-20190801/fontforge/scripting.c:10796
    #22 0x7f562306659f in _CheckIsScript /home/fcambus/fontforge-20190801/fontforge/scripting.c:10894
    #23 0x7f5623066881 in CheckIsScript /home/fcambus/fontforge-20190801/fontforge/scripting.c:10927
    #24 0x7f5624ba3be7 in fontforge_main /home/fcambus/fontforge-20190801/fontforgeexe/startui.c:1099
    #25 0x55e7ce3881ec in main /home/fcambus/fontforge-20190801/fontforgeexe/main.c:33
    #26 0x7f56242311e2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x271e2)
    #27 0x55e7ce38810d in _start (/home/fcambus/fontforge-20190801/fontforgeexe/.libs/fontforge+0x110d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/lib/x86_64-linux-gnu/libc.so.6+0x18bba4) 
==14023==ABORTING
```