Check the result of localtime().

Unlikely to fail, but it squelches CID 1398219.

Change-Id: Ibdabd2d71bdc2c09549f27f1ffe528005383ee3e
Reviewed-on: https://code.wireshark.org/review/21178
Reviewed-by: Guy Harris <guy@alum.mit.edu>

ringbuffer.c

@@ -87,6 +87,7 @@ static int ringbuf_open_file(rb_file *rfile, int *err)
   char    filenum[5+1];
   char    timestr[14+1];
   time_t  current_time;
+  struct tm *tm;
 
   if (rfile->name != NULL) {
     if (rb_data.unlimited == FALSE) {
@@ -102,13 +103,11 @@ static int ringbuf_open_file(rb_file *rfile, int *err)
   current_time = time(NULL);
 
   g_snprintf(filenum, sizeof(filenum), "%05u", (rb_data.curr_file_num + 1) % RINGBUFFER_MAX_NUM_FILES);
-  /*
-   * XXX - We trust Windows not to return a time before the Epoch, so
-   * localtime() doesn't return a null pointer.  localtime() can probably
-   * handle pre-Epoch times on most UN*X systems, and we trust them not
-   * to return a time before the Epoch in any case.
-   */
-  strftime(timestr, sizeof(timestr), "%Y%m%d%H%M%S", localtime(&current_time));
+  tm = localtime(&current_time);
+  if (tm != NULL)
+    strftime(timestr, sizeof(timestr), "%Y%m%d%H%M%S", tm);
+  else
+    g_strlcpy(timestr, "196912312359", sizeof(timestr)); /* second before the Epoch */
   rfile->name = g_strconcat(rb_data.fprefix, "_", filenum, "_", timestr,
                             rb_data.fsuffix, NULL);