segfault on: "void f(int n) {}"

[Ace17]

// yo.c
void f(int n) {}

Compilation:

$ ./9cc yo.c
[1]    27537 segmentation fault  ./9cc yo.c

Backtrace:

$ gdb --args ./9cc yo.c
Reading symbols from ./9cc...done.
(gdb) run
Starting program: /tmp/9cc/9cc yo.c

Program received signal SIGSEGV, Segmentation fault.
0x000055555555e5ab in vec_push (v=0x0, elem=0x55555556a990) at util.c:29
29        if (v->len == v->capacity) {
(gdb) bt
#0  0x000055555555e5ab in vec_push (v=0x0, elem=0x55555556a990) at util.c:29
#1  0x000055555555b728 in add_lvar (ty=0x55555556a7d0, name=0x555555569a80 "n") at parse.c:81
#2  0x000055555555d827 in param_declaration () at parse.c:717
#3  0x000055555555e0df in toplevel () at parse.c:895
#4  0x000055555555e3b5 in parse (tokens_=0x555555569fa0) at parse.c:961
#5  0x000055555555b4c4 in main (argc=2, argv=0x7fffffffe688) at main.c:34
(gdb)