Commitlog - Crash in the commitlog section

[dorlaor]

While running cassandra stress with thrift. It's really rare. However, from the IP one
can get the location of the access to null. The git commit hash is f6e17ec

[278656.991643] scylla[80105]: segfault at 0 ip 0000000000995410 sp 00007f65425f6da0 error 6 in scylla[400000+cb7000]

-sh-4.3$ gdb build/release/scylla
(gdb) disassemble 0x0000000000995410

0x000000000099540c <+444>: je 0x99541f <db::commitlog::segment::cycle(unsigned long)+463>
0x000000000099540e <+446>: xchg %ax,%ax
0x0000000000995410 <+448>: movl $0x0,(%rax)
0x0000000000995416 <+454>: add $0x4,%rax
0x000000000099541a <+458>: cmp %rax,%r15
0x000000000099541d <+461>: jne 0x995410 <db::commitlog::segment::cycle(unsigned long)+448>
0x000000000099541f <+463>: addq $0x1,(%rbx)
---Type to continue, or q to quit---
0x0000000000995423 <+467>: mov %rbx,0x20(%rsp)
0x0000000000995428 <+472>: test %r12,%r12
0x000000000099542b <+475>: jne 0x995480 <db::commitlog::segment::cycle(unsigned long)+560>
0x000000000099542d <+477>: movq $0x0,0x20(%rsp)
0x0000000000995436 <+486>: mov (%rsp),%rax

(gdb) start
Temporary breakpoint 1, main (ac=1, av=0x7fffffffe3d8) at main.cc:53
53 int main(int ac, char** av) {
(gdb) set $rip = 0x0000000000995410
(gdb) fr
#0 __fill_a<unsigned int*, int> (__value=, __last=0x0, __first=0x41c7e0 <main(int, char**)>) at /usr/include/c++/4.9.2/bits/stl_algobase.h:704

704 __first = __tmp;
(gdb) bt
#0 __fill_a<unsigned int, int> (__value=, __last=0x0, __first=0x41c7e0 <main(int, char**)>) at /usr/include/c++/4.9.2/bits/stl_algobase.h:704
#1 fill<unsigned int*, int> (__value=, __last=0x0, __first=<error reading variable: Cannot access memory at address 0x50>) at /usr/include/c++/4.9.2/bits/stl_algobase.h:739
#2 db::commitlog::segment::cycle (this=0x0, s=) at db/commitlog/commitlog.cc:309

[elcallio]

Dup of #108