Check the result of localtime().

Unlikely to fail, but it squelches CID 1398220.

Change-Id: I0e40146f0a32c1082e84052c6b3e382fe6a15ae7
Reviewed-on: https://code.wireshark.org/review/21177
Reviewed-by: Guy Harris <guy@alum.mit.edu>

wsutil/tempfile.c

@@ -170,6 +170,7 @@ create_tempfile(char **namebuf, const char *pfx, const char *sfx)
   int old_umask;
   int fd;
   time_t current_time;
+  struct tm *tm;
   char timestr[14 + 1];
   gchar *tmp_file;
   gchar *safe_pfx;
@@ -204,8 +205,11 @@ create_tempfile(char **namebuf, const char *pfx, const char *sfx)
   _tzset();
 #endif
   current_time = time(NULL);
-  /* We trust the OS not to return a time before the Epoch. */
-  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 */
   sep[0] = G_DIR_SEPARATOR;
   tmp_file = g_strconcat(tmp_dir, sep, safe_pfx, "_", timestr, "_", TMP_FILE_SUFFIX, sfx, NULL);
   g_free(safe_pfx);