[crash] Null pointer dereference occurs when using -d

[Ruanxingzhi]

When running the program with -d and doc->code is null, a null pointer dereference occurs.

This bug affects versions 2.x and 3.0.0a.

Reproduce

Run:

./markdown -d -s ''

# or feed the program with three lines of "%"

echo -ne '%\n%\n%\n' | ./markdown -d

Result:

root@worklab ~/f/d/src (main) [1]# ./markdown -d -s ''
AddressSanitizer:DEADLYSIGNAL
=================================================================
==18163==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x55f7546656fc bp 0x7ffdb5c70f10 sp 0x7ffdb5c70de0 T0)
==18163==The signal is caused by a READ memory access.
==18163==Hint: address points to the zero page.
    #0 0x55f7546656fc in mkd3_dump /root/fuzz/discount/src/dumptree.c:151:48
    #1 0x55f754649825 in main /root/fuzz/discount/src/main.c:366:11
    #2 0x7f4d3448d6c9 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #3 0x7f4d3448d784 in __libc_start_main csu/../csu/libc-start.c:360:3
    #4 0x55f7545713f0 in _start (/root/fuzz/discount/src/markdown+0x283f0) (BuildId: afc94b815b20ad8be244845d74b6694ea7bab3d9)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /root/fuzz/discount/src/dumptree.c:151:48 in mkd3_dump
==18163==ABORTING

Reason

In function mkd_dump(), doc->code->next is used without check.

Possible fix

Add a check.

[Orc]

Oh drat. Thank you for finding this annoying buglet. The code has been modified and now does the trivial test for non-null pointering before using the pointed to structure, and I've mirrored it here for
your verifying enjoyment.

[Ruanxingzhi]

Verified. Thank you for the fix!