[2019-12-13] - Found with Honggfuzz - CVE-2019-19787

Stack-based buffer overflow in the get_signed_expression() function, in
setparse.c.

Issue can be reproduced by running:

```
atasm test03
```

```
=================================================================
==10633==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffe5c1b3480 at pc 0x0000004e27ea bp 0x7ffe5c1b3210 sp 0x7ffe5c1b3208
WRITE of size 1 at 0x7ffe5c1b3480 thread T0
    #0 0x4e27e9 in get_signed_expression /home/fcambus/atasm/src/setparse.c:179:14
    #1 0x4e08a5 in get_expression /home/fcambus/atasm/src/setparse.c:154:27
    #2 0x4c8de1 in add_label /home/fcambus/atasm/src/asm.c
    #3 0x4d5563 in do_cmd /home/fcambus/atasm/src/asm.c:1934:5
    #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 0x7f1cbb2811e2 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 0x7ffe5c1b3480 is located in stack of thread T0 at offset 608 in frame
    #0 0x4e08bf in get_signed_expression /home/fcambus/atasm/src/setparse.c:157

  This frame has 4 object(s):
    [32, 288) 'err.i' (line 136)
    [352, 608) 'buf' (line 158) <== Memory access at offset 608 overflows this variable
    [672, 928) 'work' (line 158)
    [992, 1005) 'math' (line 162)
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/setparse.c:179:14 in get_signed_expression
Shadow bytes around the buggy address:
  0x10004b82e640: 00 00 00 00 f1 f1 f1 f1 f8 f8 f8 f8 f8 f8 f8 f8
  0x10004b82e650: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
  0x10004b82e660: f8 f8 f8 f8 f8 f8 f8 f8 f2 f2 f2 f2 f2 f2 f2 f2
  0x10004b82e670: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004b82e680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10004b82e690:[f2]f2 f2 f2 f2 f2 f2 f2 00 00 00 00 00 00 00 00
  0x10004b82e6a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004b82e6b0: 00 00 00 00 00 00 00 00 f2 f2 f2 f2 f2 f2 f2 f2
  0x10004b82e6c0: 00 05 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004b82e6d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004b82e6e0: 00 00 00 00 f1 f1 f1 f1 f8 f8 f8 f8 f8 f8 f8 f8
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
==10633==ABORTING
```