malloc deadlock in sigfatal_handler

Callback function sigfatal_handler called backtrace, which is not signal-safe. If a fatal signal is triggered during a malloc procedure, a deadlock happened. Here is the stack:

#0 0x00007f4f62499c0c in __lll_lock_wait_private () from /lib64/libc.so.6
#1 0x00007f4f6241618d in _L_lock_15512 () from /lib64/libc.so.6
#2 0x00007f4f62413273 in malloc () from /lib64/libc.so.6 // call malloc again, deadlock
#3 0x00007f4f64d00540 in _dl_map_object_deps () from /lib64/ld-linux-x86-64.so.2
#4 0x00007f4f64d0727b in dl_open_worker () from /lib64/ld-linux-x86-64.so.2
#5 0x00007f4f64d02754 in _dl_catch_error () from /lib64/ld-linux-x86-64.so.2
#6 0x00007f4f64d06b0b in _dl_open () from /lib64/ld-linux-x86-64.so.2
#7 0x00007f4f624caa02 in do_dlopen () from /lib64/libc.so.6
#8 0x00007f4f64d02754 in _dl_catch_error () from /lib64/ld-linux-x86-64.so.2
#9 0x00007f4f624caac2 in __libc_dlopen_mode () from /lib64/libc.so.6
#10 0x00007f4f624a1eb5 in init () from /lib64/libc.so.6
#11 0x00007f4f644b2e90 in pthread_once () from /lib64/libpthread.so.0
#12 0x00007f4f624a1fcc in backtrace () from /lib64/libc.so.6 // call backtrace
#13 0x0000000000407fcf in sigfatal_handler (i=11) at /root/MaxScale/server/core/gateway.cc:501
#14 <signal handler called> // sig fatal triggered
#15 0x00007f4f62410250 in _int_malloc () from /lib64/libc.so.6
#16 0x00007f4f62413ca4 in calloc () from /lib64/libc.so.6 // first, call calloc here
#17 0x00007f4f6497d5be in mxs_calloc (nmemb=<optimized out>, size=<optimized out>) at /root/MaxScale/server/core/alloc.cc:58
#18 0x00007f4f6497f109 in gwbuf_clone_one (buf=buf@entry=0xa89010) at /root/MaxScale/server/core/buffer.cc:304
#19 0x00007f4f6497f18a in gwbuf_clone (buf=0xa89010) at /root/MaxScale/server/core/buffer.cc:332
#20 0x00007f4f5ef1fe37 in handle_got_target (inst=inst@entry=0x9f1140, rses=rses@entry=0xa7ef10, querybuf=querybuf@entry=0xa89010, target=std::tr1::shared_ptr (count 2, weak 0) 0xa7e2d0, store=<optimized out>) at /root/MaxScale/server/modules/routing/re
adwritesplit/rwsplit_route_stmt.cc:1506
#21 0x00007f4f5ef2319f in route_single_stmt (inst=inst@entry=0x9f1140, rses=rses@entry=0xa7ef10, querybuf=querybuf@entry=0xa89010, info=...) at /root/MaxScale/server/modules/routing/readwritesplit/rwsplit_route_stmt.cc:428
#22 0x00007f4f5ef187b0 in routeQuery (instance=0x9f1140, router_session=0xa7ef10, querybuf=0xa89010) at /root/MaxScale/server/modules/routing/readwritesplit/readwritesplit.cc:904
#23 0x00007f4f5e0d28df in route_by_statement (p_readbuf=<optimized out>, capabilities=<optimized out>, session=<optimized out>) at /root/MaxScale/server/modules/protocol/MySQL/mariadbclient/mysql_client.cc:1845
#24 gw_read_finish_processing (capabilities=<optimized out>, read_buffer=0x0, dcb=<optimized out>) at /root/MaxScale/server/modules/protocol/MySQL/mariadbclient/mysql_client.cc:1200
#25 gw_read_normal_data (nbytes_read=<optimized out>, read_buffer=0xa89010, dcb=<optimized out>) at /root/MaxScale/server/modules/protocol/MySQL/mariadbclient/mysql_client.cc:1142
#26 gw_read_client_event (dcb=<optimized out>) at /root/MaxScale/server/modules/protocol/MySQL/mariadbclient/mysql_client.cc:546
#27 0x00007f4f649949c9 in dcb_process_poll_events (dcb=0xa7de10, events=5) at /root/MaxScale/server/core/dcb.cc:3242
#28 0x00007f4f64994c5b in dcb_handler (dcb=0xa7de10, events=5) at /root/MaxScale/server/core/dcb.cc:3329
#29 0x00007f4f649d54dd in maxscale::Worker::poll_waitevents (this=this@entry=0x96e840) at /root/MaxScale/server/core/worker.cc:1235
#30 0x00007f4f649d5773 in maxscale::Worker::run (this=0x96e840) at /root/MaxScale/server/core/worker.cc:901
#31 0x0000000000406af5 in main (argc=<optimized out>, argv=<optimized out>) at /root/MaxScale/server/core/gateway.cc:2276