Illegal Instruction crash on startup with spread_checks in src/checks.c

haproxy crashes after starting up successfully, when the spread_checks option is provided.

According to GDB, here is the backtrace of the crash:
```
Program received signal SIGILL, Illegal instruction.
0x000000000042b97f in process_chk (t=0x6ef7c0) at src/checks.c:1587
1587 src/checks.c: No such file or directory.
 in src/checks.c
(gdb) bt
#0 0x000000000042b97f in process_chk (t=0x6ef7c0) at src/checks.c:1587
#1 0x00000000004103ee in process_runnable_tasks (next=0x7fffffffe4cc) at src/task.c:240
#2 0x0000000000406440 in run_poll_loop () at src/haproxy.c:1304
#3 0x0000000000408966 in main (argc=<value optimized out>, argv=0x7fffffffe6f8) at src/haproxy.c:1638
```

it's caused by this code:
```
  if (global.spread_checks > 0) {
   rv = srv_getinter(check) * global.spread_checks / 100;
   rv -= (int) (2 * rv * (rand() / (RAND_MAX + 1.0)));
  }
```
on line 1587 of src/checks.c (the second line in the if clause).

We're running haproxy 1.5.0

> lsb_release -rd
Description: Ubuntu 10.04.4 LTS
Release: 10.04

> uname -a
Linux 789b4b1b-6f7b-44cf-accc-88d90341f17a 3.8.0-29-generic #42~precise1-Ubuntu SMP Wed Aug 14 16:19:23 UTC 2013 x86_64 GNU/Linux