Heap Use After Free in the cmpstring() function

Hi,

While fuzzing dpic 2021.04.10 (commit: d317e406), I found a heap use-after-free in the cmpstring() function, in dpic.y:L4059.

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.2 LTS"

Attaching a reproducer: heap-uaf-read-test-cmpstring

the issue can be reproduced by running:

dpic heap-uaf-read-test-cmpstring

=================================================================
==3305695==ERROR: AddressSanitizer: heap-use-after-free on address 0x60d000000048 at pc 0x00000051a66b bp 0x7ffe3d0264e0 sp 0x7ffe3d0264d8
READ of size 8 at 0x60d000000048 thread T0
    #0 0x51a66a in cmpstring /home/bsdboy/projects/again/dpic/dpic.y:4059:16
    #1 0x4fd41a in yyparse /home/bsdboy/projects/again/dpic/dpic.y:2862:8
    #2 0x4d69ac in main /home/bsdboy/projects/again/dpic/main.c:1813:13
    #3 0x7f90681400b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #4 0x41c3dd in _start (/home/bsdboy/projects/again/dpic/dpic+0x41c3dd)

0x60d000000048 is located 8 bytes inside of 136-byte region [0x60d000000040,0x60d0000000c8)
freed by thread T0 here:
    #0 0x4973d2 in free (/home/bsdboy/projects/again/dpic/dpic+0x4973d2)
    #1 0x501778 in deletetree /home/bsdboy/projects/again/dpic/dpic.y:3917:12
    #2 0x50989b in deletestringbox /home/bsdboy/projects/again/dpic/dpic.y:5732:3
    #3 0x4fce1a in yyparse /home/bsdboy/projects/again/dpic/dpic.y:2839:4
    #4 0x4d69ac in main /home/bsdboy/projects/again/dpic/main.c:1813:13
    #5 0x7f90681400b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16

previously allocated by thread T0 here:
    #0 0x49763d in malloc (/home/bsdboy/projects/again/dpic/dpic+0x49763d)
    #1 0x507b27 in newprim /home/bsdboy/projects/again/dpic/dpic.y:4984:13
    #2 0x4deb1a in yyparse /home/bsdboy/projects/again/dpic/dpic.y:892:19
    #3 0x4d69ac in main /home/bsdboy/projects/again/dpic/main.c:1813:13
    #4 0x7f90681400b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16

SUMMARY: AddressSanitizer: heap-use-after-free /home/bsdboy/projects/again/dpic/dpic.y:4059:16 in cmpstring
Shadow bytes around the buggy address:
  0x0c1a7fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c1a7fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c1a7fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c1a7fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c1a7fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c1a7fff8000: fa fa fa fa fa fa fa fa fd[fd]fd fd fd fd fd fd
  0x0c1a7fff8010: fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa fa
  0x0c1a7fff8020: fa fa 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c1a7fff8030: 00 00 00 fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff8050: 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
==3305695==ABORTING