Segmentation fault while using String.Buffer.

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=2845

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

#0  f_Buffer_add (args=3) at /home/nilsson/Pike/7.3/src/builtin.cmod:1458
1458            sum += a->len;
(gdb) bt
#0  f_Buffer_add (args=3) at /home/nilsson/Pike/7.3/src/builtin.cmod:1458
#1  0x0b713f04 in ?? ()
#2  0x080713ee in low_mega_apply (type=APPLY_LOW, args=3, arg1=0xb713f04,
arg2=0x5) at /home/nilsson/Pike/7.3/src/apply_low.h:187
#3  0x0806fbfb in opcode_F_CALL_OTHER_AND_POP (arg1=98) at
/home/nilsson/Pike/7.3/src/interpret_functions.h:1888
#4  0x08360cb1 in ?? ()
#5  0x080726c0 in mega_apply (type=APPLY_LOW, args=2, arg1=0x82a0e68,
arg2=0x51) at /home/nilsson/Pike/7.3/src/interpret.c:1402
#6  0x080731a7 in apply (o=0x82a0e68, fun=0x8192317 "_main", args=2) at
/home/nilsson/Pike/7.3/src/interpret.c:1703
#7  0x080c6bc1 in main (argc=4, argv=0xbffffacc) at
/home/nilsson/Pike/7.3/src/main.c:716
#8  0x400c7306 in __libc_start_main (main=0x80c6230 <main>, argc=4,
ubp_av=0xbffffacc, init=0x805f88c <_init>,
    fini=0x8172590 <_fini>, rtld_fini=0x4000d2dc <_dl_fini>,
stack_end=0xbffffabc) at ../sysdeps/generic/libc-start.c:129