Fix null pointer dereference.

In the function dump_thread_info, if /proc/<PID>/comm is unreadable, then
threadname will be used in a strncmp causing a crash. The fix is to
avoid the check if threadname is null.

Bug: 28615417
(cherry picked from commit 039976e)

Change-Id: I1e61431b1549ecfdc49a72cbf193a364069cda79

debuggerd/tombstone.cpp

@@ -201,7 +201,7 @@ static void dump_signal_info(log_t* log, pid_t tid, int signal, int si_code) {
 static void dump_thread_info(log_t* log, pid_t pid, pid_t tid) {
   char path[64];
   char threadnamebuf[1024];
-  char* threadname = NULL;
+  char* threadname = nullptr;
   FILE *fp;
 
   snprintf(path, sizeof(path), "/proc/%d/comm", tid);
@@ -217,13 +217,13 @@ static void dump_thread_info(log_t* log, pid_t pid, pid_t tid) {
   }
   // Blacklist logd, logd.reader, logd.writer, logd.auditd, logd.control ...
   static const char logd[] = "logd";
-  if (!strncmp(threadname, logd, sizeof(logd) - 1)
+  if (threadname != nullptr && !strncmp(threadname, logd, sizeof(logd) - 1)
       && (!threadname[sizeof(logd) - 1] || (threadname[sizeof(logd) - 1] == '.'))) {
     log->should_retrieve_logcat = false;
   }
 
   char procnamebuf[1024];
-  char* procname = NULL;
+  char* procname = nullptr;
 
   snprintf(path, sizeof(path), "/proc/%d/cmdline", pid);
   if ((fp = fopen(path, "r"))) {