null pointer deference in function gchord_width() in music.c

[chibataiki]

Hi ,
Null pointer deference was found in function gchord_width() in music.c.

version :
9fa642d

env:
ubuntu 20.04 x86_64
gcc version 9.3.0

reproduce:
./configure
make
./abcm2ps poc

null_pointer_music.c_gchord_width_880.zip

Here didn't check whether s2 is valid, add check here or the for loop above or more deeper.

debug info

─ source:music.c+880 ────
    875	 			for (s2 = s->ts_next; ; s2 = s2->ts_next) {
    876	 				if (s2 == s->next) {
    877	 					AT_LEAST(s->wr, rspc);
    878	 					break;
    879	 				}
 →  880	 				if (s2->sflags & S_SEQST)
    881	 					rspc -= 8;
    882	 			}
    883	 		}
    884	 		if (arspc != 0)
    885	 			AT_LEAST(s->wr, arspc);
── threads ────
[#0] Id 1, Name: "abcm2ps", stopped 0x55555557cbd8 in gchord_width (), reason: SIGSEGV
─── trace ────
[#0] 0x55555557cbd8 → gchord_width(s=0x55555560bd28, wlnote=8, wlw=8)
[#1] 0x555555580557 → set_width(s=0x55555560bd28)
[#2] 0x5555555809b4 → set_allsymwidth(last_s=0x0)
[#3] 0x555555583833 → output_music()
[#4] 0x55555558a177 → generate()
[#5] 0x55555558a177 → generate()
[#6] 0x55555558a5f8 → gen_ly(eob=0x0)
[#7] 0x555555590918 → do_tune()
[#8] 0x55555555eb7d → abc_eof()
[#9] 0x55555557ae6c → frontend(s=<optimized out>, ftype=<optimized out>, fname=<optimized out>, linenum=0xf)

[chibataiki]

Tried and fix, thanks for the work .