debuggerd: fix `debuggerd -b <pid>` am: b53f903cf1 am: b5be6e3017
am: 48e5ce8bd3

Change-Id: Idce33914d00ef4b8e55cd73f99dede7181826326
diff --git a/debuggerd/crash_dump.cpp b/debuggerd/crash_dump.cpp
index c145933..58eaed7 100644
--- a/debuggerd/crash_dump.cpp
+++ b/debuggerd/crash_dump.cpp
@@ -322,7 +322,7 @@
   bool fatal_signal = signo != DEBUGGER_SIGNAL;
   std::set<pid_t> siblings;
   std::set<pid_t> attached_siblings;
-  if (fatal_signal) {
+  if (fatal_signal || backtrace) {
     if (!android::procinfo::GetProcessTids(target, &siblings)) {
       PLOG(FATAL) << "failed to get process siblings";
     }