CRASH from race on detach in LOG debug-build code

While testing a fix for #3535 (closed), running debug build I hit this crash maybe once in 100 runs:

#4  0x00007fe6e8f590a5 in getchar () at getchar.c:37
#5  0x00005640f8054db3 in handle_signal (signal=11, siginfo=0x7fe6e56cff30, ucxt=0x7fe6e56cfe00) at /home/bruening/dr/git/src/suite/tests/api/detach_signal.cpp:105
#6  <signal handler called>
#7  0x00007fe6e93d1c72 in master_signal_handler_C (sig=7, siginfo=0x7fe6e56d0770, ucxt=0x7fe6e56d0640, xsp=0x7fe6e56d0638 ";\016\071\351\346\177") at /home/bruening/dr/git/src/core/unix/signal.c:5107

   0x00007fe6e93d1c59 <+1004>:	lea    0x161388(%rip),%rax        # 0x7fe6e9532fe8 <d_r_stats>
   0x00007fe6e93d1c60 <+1011>:	mov    (%rax),%rax
   0x00007fe6e93d1c63 <+1014>:	test   %rax,%rax
   0x00007fe6e93d1c66 <+1017>:	je     0x7fe6e93d1ccc <master_signal_handler_C+1119>
   0x00007fe6e93d1c68 <+1019>:	lea    0x161379(%rip),%rax        # 0x7fe6e9532fe8 <d_r_stats>
   0x00007fe6e93d1c6f <+1026>:	mov    (%rax),%rax
==>
   0x00007fe6e93d1c72 <+1029>:	mov    0x218(%rax),%eax
   0x00007fe6e93d1c78 <+1035>:	test   %eax,%eax
   0x00007fe6e93d1c7a <+1037>:	je     0x7fe6e93d1ccc <master_signal_handler_C+1119>
   0x00007fe6e93d1c7c <+1039>:	lea    0x161365(%rip),%rax        # 0x7fe6e9532fe8 <d_r_stats>
   0x00007fe6e93d1c83 <+1046>:	mov    (%rax),%rax
   0x00007fe6e93d1c86 <+1049>:	mov    0x214(%rax),%eax
   0x00007fe6e93d1c8c <+1055>:	and    $0x10,%eax
   0x00007fe6e93d1c8f <+1058>:	test   %eax,%eax
   0x00007fe6e93d1c91 <+1060>:	je     0x7fe6e93d1ccc <master_signal_handler_C+1119>
   0x00007fe6e93d1c93 <+1062>:	callq  0x7fe6e93ae109 <get_sys_thread_id>

(gdb) p &((dr_statistics_t*)0)->loglevel
$2 = (uint *) 0x218
(gdb) p d_r_stats
$3 = (dr_statistics_t *) 0x0

Must be a race where d_r_stats is set to NULL in between the check for NULL and the de-ref of loglevel.

I was not planning to fix it: b/c it would require eliminating all LOG calls on detach paths, which are useful for debugging, and b/c it is limited to debug build.