[2019-12-13] - Found with Honggfuzz - CVE-2019-19785

Stack-based buffer overflow in the to_comma() function, in asm.c.

Issue can be reproduced by running:

```
atasm test01
```

```
=================================================================
==15033==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffec9190ef0 at pc 0x0000004ce38e bp 0x7ffec9190e30 sp 0x7ffec9190e28
WRITE of size 1 at 0x7ffec9190ef0 thread T0
    #0 0x4ce38d in to_comma /home/fcambus/atasm/src/asm.c:1126:11
    #1 0x4ce38d in do_xbyte /home/fcambus/atasm/src/asm.c:1346:9
    #2 0x4cfe17 in proc_sym /home/fcambus/atasm/src/asm.c:1553:7
    #3 0x4d5556 in do_cmd /home/fcambus/atasm/src/asm.c:1941:5
    #4 0x4d5b46 in assemble /home/fcambus/atasm/src/asm.c:1980:9
    #5 0x4d8082 in main /home/fcambus/atasm/src/asm.c:2392:3
    #6 0x7f32f46441e2 in __libc_start_main /build/glibc-4WA41p/glibc-2.30/csu/../csu/libc-start.c:308:16
    #7 0x41b3fd in _start (/home/fcambus/atasm/atasm+0x41b3fd)

Address 0x7ffec9190ef0 is located in stack of thread T0 at offset 176 in frame
    #0 0x4cc7af in do_xbyte /home/fcambus/atasm/src/asm.c:1299

  This frame has 2 object(s):
    [32, 64) 'buf.i' (line 736)
    [96, 176) 'buf' (line 1301) <== Memory access at offset 176 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /home/fcambus/atasm/src/asm.c:1126:11 in to_comma
Shadow bytes around the buggy address:
  0x10005922a180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10005922a190: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10005922a1a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10005922a1b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10005922a1c0: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 f8 f8 f8 f8
=>0x10005922a1d0: f2 f2 f2 f2 00 00 00 00 00 00 00 00 00 00[f3]f3
  0x10005922a1e0: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x10005922a1f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10005922a200: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 f8 f8 f8 f8
  0x10005922a210: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
  0x10005922a220: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f2 f2 f2 f2
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
==15033==ABORTING
```