[2019-12-10] - Found with Honggfuzz - CVE-2019-19720

Heap-based buffer overflow in the yylex() function, in flex.c.

Issue can be reproduced by running:

```
yabasic test01.yab
```

```
=================================================================
==20945==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x603000000223 at pc 0x0000005a1a77 bp 0x7ffd03025cb0 sp 0x7ffd03025ca8
WRITE of size 1 at 0x603000000223 thread T0
    #0 0x5a1a76 in yylex /home/fcambus/yabasic-2.86.1/flex.c:3100:27
    #1 0x5aad98 in yyparse /home/fcambus/yabasic-2.86.1/bison.c:3338:16
    #2 0x4f9497 in main /home/fcambus/yabasic-2.86.1/main.c:293:6
    #3 0x7f5abf9cab6a in __libc_start_main /build/glibc-KRRWSm/glibc-2.29/csu/../csu/libc-start.c:308:16
    #4 0x41da49 in _start (/home/fcambus/yabasic-2.86.1/yabasic+0x41da49)

Address 0x603000000223 is a wild pointer.
SUMMARY: AddressSanitizer: heap-buffer-overflow /home/fcambus/yabasic-2.86.1/flex.c:3100:27 in yylex
Shadow bytes around the buggy address:
  0x0c067fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff8000: fa fa 00 00 00 07 fa fa 00 00 00 04 fa fa 00 00
  0x0c067fff8010: 00 fa fa fa fd fd fd fa fa fa 00 00 00 fa fa fa
  0x0c067fff8020: fd fd fd fa fa fa 00 00 00 fa fa fa 00 00 00 fa
  0x0c067fff8030: fa fa 00 00 01 fa fa fa 00 00 00 05 fa fa fa fa
=>0x0c067fff8040: fa fa fa fa[fa]fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8070: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8080: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8090: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==20945==ABORTING
```