Heap-based Buffer Overflow in the storestring() function

Hi,

While fuzzing dpic 68ab9432 with Hongfuzz, I found a Heap-based Buffer Overflow in the storestring() function, in dpic.y.

Attaching a reproducer, the issue can be reproduced by running: test02

dpic test02

=================================================================
==1522230==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x621000006100 at pc 0x000000527df7 bp 0x7ffcc985f3d0 sp 0x7ffcc985f3c8
WRITE of size 1 at 0x621000006100 thread T0
    #0 0x527df6 in storestring /src/dpic/dpic.y:4266:51
    #1 0x4fac82 in yyparse /src/dpic/dpic.y:911:5
    #2 0x4e67b3 in main /src/dpic/main.c:1795:13
    #3 0x7f3bdc9fa0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #4 0x41c48d in _start (/src/dpic/dpic+0x41c48d)

0x621000006100 is located 0 bytes to the right of 4096-byte region [0x621000005100,0x621000006100)
allocated by thread T0 here:
    #0 0x4976ed in malloc (/src/dpic/dpic+0x4976ed)
    #1 0x5273c4 in storestring /src/dpic/dpic.y:4260:15

SUMMARY: AddressSanitizer: heap-buffer-overflow /src/dpic/dpic.y:4266:51 in storestring
Shadow bytes around the buggy address:
  0x0c427fff8bd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c427fff8be0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c427fff8bf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c427fff8c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c427fff8c10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c427fff8c20:[fa]fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c427fff8c30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c427fff8c40: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c427fff8c50: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c427fff8c60: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c427fff8c70: 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
==1522230==ABORTING