mixed x={} dumps core

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=3278

Reported by Martin Nilsson, IDA nilsson@pike.ida.liu.se

bin/pike --gdb -e "mixed x={}"

gives

-:1:parse error

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 1024 (LWP 28011)]
recursive_add_call_arg (n=0x0, arg=0x82d1518)
    at /home/nilsson/Pike/7.3/src/las.c:1714
1714      switch(n->token)
(gdb) bt
#0  recursive_add_call_arg (n=0x0, arg=0x82d1518)
    at /home/nilsson/Pike/7.3/src/las.c:1714
#1  0x0806358c in yyparse () at /home/nilsson/Pike/7.3/src/language.yacc:1777
#2  0x080f11b5 in run_pass1 (c=0x82f0770)
    at /home/nilsson/Pike/7.3/src/program.c:5466
#3  0x080f17ed in compile (aprog=0x82f0650, ahandler=0x0, amajor=-1,
    aminor=-1, atarget=0x0, aplaceholder=0x0)
    at /home/nilsson/Pike/7.3/src/program.c:5731
#4  0x08122691 in f_compile (args=4)
    at /home/nilsson/Pike/7.3/src/builtin_functions.c:3225
#5  0x080702d8 in opcode_F_CALL_BUILTIN (arg1=15)
    at /home/nilsson/Pike/7.3/src/interpret_functions.h:2050
#6  0x083203f8 in ?? ()
#7  0x08072b00 in mega_apply (type=APPLY_LOW, args=2, arg1=0x82a0618,
    arg2=0x52) at /home/nilsson/Pike/7.3/src/interpret.c:1431
#8  0x08073657 in apply (o=0x82a0618, fun=0x8198077 "_main", args=2)
    at /home/nilsson/Pike/7.3/src/interpret.c:1734
#9  0x080ca5d1 in main (argc=4, argv=0xbffffa8c)
    at /home/nilsson/Pike/7.3/src/main.c:726
#10 0x400c7336 in __libc_start_main (main=0x80c9c30 <main>, argc=4,
    ubp_av=0xbffffa8c, init=0x805f96c <_init>, fini=0x81771d0 <_fini>,
    rtld_fini=0x4000d2fc <_dl_fini>, stack_end=0xbffffa7c)
    at ../sysdeps/generic/libc-start.c:129