libutils: Don't segfault ProcessCallStack when getting thread name fails

There's an inherent race in trying to read out the thread name from
the system and the thread closing out on its own (and thus being removed
from procfs).

Try to handle this by formatting the thread name unconditionally with
the tid when getting the thread name fails (instead of dereferencing
NULL and crashing).

Bug: 15406837
Change-Id: Ibf2208b8ce345589e7e9c57e6a307728d1121c5d

libutils/ProcessCallStack.cpp

@@ -90,6 +90,11 @@ static String8 getThreadName(pid_t tid) {
         ALOGE("%s: Failed to open %s", __FUNCTION__, path);
     }
 
+    if (procName == NULL) {
+        // Reading /proc/self/task/%d/comm failed due to a race
+        return String8::format("[err-unknown-tid-%d]", tid);
+    }
+
     // Strip ending newline
     strtok(procName, "\n");